There are some games that are hard to get legally, like old games that have been abandoned by their original developers, or are in copyright limbo. However, if you do manage to get said games, it's still possible to play them (though it may be necessary to crack DRM to do so in some cases).

There are even some games that can only be obtained through illegal methods, as the legal source of said games involves an online download that is no longer available. (For example, the game is delisted, or the service has shut down.) If you somehow obtain said game, you can play it, though it likely won't be legal.

But then, there are some games that have been completely lost. In these cases, it's impossible to obtain the game (because no copies exist anywhere), or it may be possible to get the game but not play it (the hardware no longer exists and hasn't been emulated), or the game might depend on an online service to function that is no longer available (like if the game is an MMO).

Some examples of lost games that I know of:
* Tennis for Two (game ran on some system that used an oscilloscope for its display, note that a simulation of the game exists at least)
* Dark Sun Online: Crimson Sands (a defunct MMO)
* Darkspore (game that required a constant connection to a server, and the server is shut down)

So, what other lost games do you know of? Are there any you would like to play?

There are tons of MMOs that have been discontinued. I think one of the first was neverwinter in its original form. I don't know if I would be interested in playing any. I more want to play games in the first category that you stated. That you can get the media, but it is not on any digital storefront. So I am more in the camp of wanting to play difficult to play games
(or illegal) as opposed to the impossible to play ones. I think I did actually find an actual copy of all of those games that I wanted now. So that is good.

I think you can also make a case that some games are no longer the game they were and it would be difficult/impossible to find the original to play anymore even if you wanted to. No Mans Sky is not really the same game that it was when it released. I don't know if people would want to go back or not anymore, but I think doing so would be rather difficult. There are many other games in this category as well.

Like Skyrim 1.2? (This version is notable for having bugs not found in any other version; it's of historical note because Steam forced this update on players, breaking things that previously worked (like elemental resistance), and it's the reason that Steam (and therefore GOG Galaxy) has the beta channel.) I wouldn't recommend this version to a casual player, but a video game historian might want to play it.

Back to the main topic, I thnk that I would be interested in playing Darkspore, if there were a way to play it.

Limbo of the Lost
"The studio initially aimed to bring back the graphical text adventure genre on the Atari ST, and later the Amiga 500, culminating in a demo shown at the ECTS in 1995. Due to sinking demand of these platforms, the game was instead redeveloped for the PC and released in March 2008. In June 2008, discovery of plagiarism in the game led to its withdrawal from sale."

It looks like that game is possible to acquire and play, either from getting a copy during the brief time it was available for sale, or through methods of questionable legality. Then again, it appears that the creation of the game is legally questionable due to the use of assets from commercial games, presumably without permission.

There's actually a website dedicated to stuff like this:-

https://lostmediawiki.com/Home

In many cases it's a console build that was made but then cancelled, or an online game. There are some interesting lost pre-alpha builds of cancelled games though including Doom 4 (not the 2016 one, but the one that was cancelled in 2011 in the same way Prey 2 was cancelled and replaced with Prey (2017)) and the early prototype of Myst 4 Adventure Beyond the D'ni Ultraworld (which some fans would rather have had than Ubisoft Montreal's version).

I don't think the second option actually makes the game "lost". Sure, you can't play it NOW, but sooner or later someone will make an emulation system that the game will be able to run.

The third option is either a sub-variant of the option (1) if the server code is completely lost and non-replecatable. Or it's again (2) not lost - if server code exists and it's just no one had made a server emulation yet.

games die, especially bad games
get over with it

This is why I wish they would make single player versions for some of these MMOs.

I loved Guild Wars back in the day, but the online parts I just don't have the time anymore.

which one is that?
just imagine how many mmos could have been made and you missed all of them :O

Is a game considered lost if technical issues make it infeasible to run on modern systems? If yes, I'd like to qualify Gunmetal.

Second question: Do tech demos turned games count? If yes, I'd like to qualify Voxar's Tumblebugs. Not the Tumblebugs listed on Mobygames, this was an obscure [url=http://vcg.isti.cnr.it/~tarini/index.php?3?tumble_bugs.html]tank combat game.[/url]

Third runner up; due to the publisher still existing (and even adding games here): Rails Across America, a sort of digital board game about trying to conquer rail scenarios.

who cares about these games?:O especially tech demos ... makes 0 sense , total useless info
just forget them ever existed

While I don't make it a habit to reply to you for good reason, I'll just sit you down and remind you that technological innovation is important. No, it doesn't make a lot of sense to cram a full polygonal game onto the Game Boy, but they did it. Showing what can be done, especially beyond the application of the system it was designed for can inspire others.

This site could also be of interest: https://www.gamesthatwerent.com/
Some of those games are still available in one way or another, and some others don't actually ever existed because they were canceled at a very early stage, but there are also some that fit the premise of the thread.