On GOG. A lot of them sadly aren't.

Castlevania 2 is a metroidvania? I tried to play a more modern one, Rondo of Blood, but couldn't, it was very arcadey and linear for me to view it as such a game. Symphony of the Night was definitely one, as it was influenced by Super Metroid.

I think some fellow here told me once that Iconoclasts is rather linear, which also changed my mind about getting it. It's the same thing I felt with Owlboy, some people made it look like a MV but it was 100% linear, with the illusion of an "open" world.

I'm definitely going to get Alwa series someday as well!

Oh, and if you haven't tried it, I can recommend Shantae Half Genie Hero for the female protagonist. It may be structured in "levels" (contrary to the rest of the series) but if you play it you'll find out it is pretty much a MV. I haven't played the rest, but for this specific entry trust me it's a great game :) Unfortunately the WayForward games don't get impressive discounts lately.

Back in the day, it wasn't unusual for the second game in the series to be very different from the other games in the series.

For example, Super Mario Bros. 2 (USA) is very different; you don't get power ups, and you don't kill enemies by stomping on them, but you can pick up enemies and throw them at other enemies to kill them. Also, all the enemies were new to the series, and the only one to appear in SMB3 (which is basically SMB1 but with a lot more stuff) are the bob-ombs.

We see something similar happening with some Japanese RPGs. Final Fantasy 2 uses a classless, XP-less, system for character growth, where your actions affect your stat growth at the end of the battle. Fire Emblem Gaiden has out-of-combat exploration, branched promotions and promotion loops, monsters as enemies, no durability (with spells costing HP), and there might even be repeatible encounters (note that I haven't played it myself).

The Ys series did something similar, though not until the 3rd game. Ys 2 is a continuation of Ys 1, but Ys 3 is very different, being a side-scroller.

So, it's not that surprising that Castlevania 2 is very different from the other Castlevania games of its era. You have a (mostly) continuous world to explore instead of a linear sequence of levels, there are towns and permanent items (including permanent whip upgrades and quest items), and there's even a very basic system where you can level up by earning experience. There's also a day/night cycle; at night, outdoor enemies are stronger, and if you take too long to finish the game (measured in in-game days), you'll get a worse ending.

Castlevania 2 may have been influenced by the original Metroid (specifically with the endings being time-based), but we also see some similarities to Knightmare II: Maze of Galious, released the same year by the same company, in the use of an overworld with dungeon sub-worlds (though it feels like Castlevania 2 puts more focus on the overworld). Even then, we see some Zelda 1 influence in these games.

(Note that Castlevania 1 and 3 are not metroidvanias, but Castlevania 2 could be seen as one.)

Super Mario Bros 2 is actually my favorite Mario game :) I had bought all of them back then for my NES but this one stood out for me (and yeah I know the story that it wasn't intended to be a Mario game etc.). Thanks for reminding me that I should play it again after all these years!

As for Castlevania, I see. I admit I'm not a super fan or anything, even though I knew the games from the NES era. I have only played Symphony Of The Night recently and enjoyed it very much. It came as a pack with Rondo Of Blood (that's how I got it on the PS Store) so I was super excited to play both, but Rondo just... didn't click for me. I think I could give it a go again if it wasn't for the movement restrictions when you hit (you extend your whip but can't react to anything else until the animation ends). Even if that wasn't the case, I wouldn't consider it a metroid-like.

Also, really, if you haven't played any of the Shantae games you should give them a try, I think you will be surprised.

The approach to world structure differed across various games considered as platform adventure or metroidvania (which wasn't used until 2003-ish) back in the day and now too, such as the separate overworld in Zelda 2 or Gargoyle's Quest, hub map in Clash at Demonhead (later used in Shaman King for GBA), or hub area/same engine overworld in Maze of Galious, CV2, MW series etc. This last one is continued in Ori 1 and Monster Boy (overworld and dungeons split like in Zelda) and to an extent in Hollow Knight and Dawn of Sorrow.

Various new and old games considered MVs are also linear or near linear such as CV2, Maze of Galious, Cave Story, Metroid 2, Metroid Fusion, Ori 1, Dawn of Sorrow. I'm documenting this and other typical features in this spreadsheet:
https://docs.google.com/spreadsheets/d/1nA12FiqLUe1xoRyjXhaQYOMGpzylhejqsFek148-BGs/edit#gid=0

There are a lot of female protag games even since the early days of games:
https://www.mobygames.com/game-group/protagonist-female/offset,2275/so,1d/

controversialy, the best metroidvania of 2020 was Journey to the savage planet

All metroidvanias (at least the way I define the genre) are platformers, but not all platformers are metroidvanias.

Basically, metroidvanias typically have the following traits that are not usually present in other platformers:
* The areas of the game are connected in various ways; you don't have completely separate stages. Often (like in Super Metroid but not Metroid: Zero Mission) it is possible to have a single map containing the entire game world. (If you try this in M:ZM, I believe Crateria will intersect Brinstar.) (This is in contrast to the stage structure, where you play one stage, beat it, then go on to the next without any way to go back.)
* The world branches off in different directions, so there's some exploration to be done. Also, the game usually lets you backtrack to earlier areas.
* Scattered through the game world are various upgrades. These can range from minor upgrades (like missile tanks in Metroid that increase your missile capacity by 5) to major ones (like those that grant new movement abilities).
* Sometimes, you'll see an area, but not be able to reach it because you haven't found the required movement ability to reach it. In some cases, the movement ability might not come until much later in the game, and you will have to backtrack to use this. (The resulting area may be mandatory, or it may just have some minor upgrades.)
* RPG-style leveling is present in some metroidvanias (including Castlevania: Symphony of the Night) but not in others (Metroid).

The genre really did originate on consoles. Metroid was a Famicom Disk System game (that was ported to cartridge for its overseas release, with the save system replaced with password saves), while Castlevania: Symphony of the Night was originally a PlayStation game.

It was after Castlevania: SotN that the name was established. People noticed some similarities between that game and Super Metroid, so they just mashed the games' names together to get the "metroidvania" term.

Castlevania 2 came before the Metroid series had gotten fully established; it was a while before Metroid 2 (which is more linear) came out, and Castlevania 3 went back to its Castlevania 1 formula. (I personally wish CV3 were an improved CV2 rather than an improved CV1; I would have probably beaten it if that were the case, and there could have been some interesting exploration elements with the CV3 spirit helpers.)

The term wasn't used until around 2003, when it was first used in a published review. It also wasn't well defined by the people who popularized it, Jeremy Parish for example thought 3D Zelda was the same thing as late as 2008.

https://www.youtube.com/watch?v=Rdiv5W9xrQM

https://web.archive.org/web/20040823231430/http://archive.gamespy.com/reviews/may03/castlevaniaaosgba/

I don't know what you mean by fully established, Metroid 1 was a hit in the west and Metroid 2 was popular as well though not on the same level. However the point is that the overarching structure can be mostly linear and the game still be a MV. It's really just certain parts of the fandom that think every game should be as open as la mulana or hollow knight.

Actually, regarding the structure of the 2D Metroid games, they're not all the same:
* Metroid 1: Not linear, except that you need to beat the 2 other bosses before you can go to where Mother Brain is (unless you're willing to do a tricky and probably unintentional damage boost to get to Tourian early).
* Metroid 2: Globally linear; you can backtrack but there's little reason to. With that said, each individual sub-area is non-linear. Note that even 100% doesn't require backtracking between areas.
* Super Metroid: There's an intended order in which to do things, but there are some advanced techniques (including 2 that the game teaches you) that allow you to sequence break. In fact, it's possible, but difficult, to do the second to last boss (Ridley) first (though doing Maridia right after that isn't possible without a glitch, so Reverse Boss Order (which is well-defined) can't be done completely glitchless).
* Metroid Fusion: Strictly linear. While the world has a hub structure, the game will only let you go one place at a time, and you can't backtrack unless the game requires it. (Exception: At the end there's an optional clean-up phase where you can explore much of the game world freely to reach 100%.)
* Metroid: Zero Mission: The game explicitly shows you where you need to go, but there are intentional sequence breaks that allow you to do things out of order to an extent. However, the new area that's added can't be reached without killing Mother Brain first, unless there's a glitch I don't know about; that new area gives new power-ups that are required for some earlier ones, so the game again has a clean-up phase.
* (Haven't played Samus Returns on the 3DS, but it appears to be structurally similar to Metroid 2, but possibly with some mionr treasures that require backtracking, and with the endgame cleanup phase as well.)

By fully established, I mean that Super Metroid is the first game to feel like a modern entry in the series, with an automap (now a standard aspect of the genre), and the earliest game in the series that modern metroidvanias tend to be compared to.

Oh, and Hollow Knight doesn't really open up until you get the mantis claw (though once you get dash there are 2 paths to the next place you need to go).

^I know, I just told you and I also linked to you my thorough documentation of all 80s-90s games in the genre.

HK opens up after the first sub area below the town, you can also buy the lantern at that point.

Why would the automap feature be the line between modern and not modern here? Furthermore it isn't related to a series being fully established, or important to the definition of the genre.

I can tell you one thing I don't like: When "brutal platforming" is your progress block (rather than exploration or getting a new item). There's a difference between "challenging platforming" and "brutal platforming". It may vary from player to player a bit, but the developers should know when they've pushed it too far.

Hollow Knight [White Palace -- most all of it, not just the small optional side area] and Guacamelee [Tule Tree Tops] know I'm pointing at them...

I can say I did complete both. Eventually. Begrudgingly.

I second that.

Hollow Knight disappointed me for many reasons (mostly brutal checkpoints + bosses combination) so I did not bother finishing it. I wish the sequel has at least some option to balance these issues for non-darksoulers like me, as I liked the game in all other aspects.

Guacamelee was fine (a bit too much combat-focused for my taste but manageable nonetheless) and if I remember correctly those uber hardcore platforming sections were for optional prizes. I still haven't played the sequel though.

By the way, the number of metroidvanias on GOG is impressively low. We need more!