I think it is striking that the new version got even updated now for Windows and Mac,
while GNU/Linux is still on the initial version - not having the announced update yet.

And for the sequel the same game ... currently only Windows version is given to come.
GOG is clearly on the road to no longer provide Linux users updates or new releases - even when those are on Steam.

I'm not sure why the devs or publisher decided to add "Highlander Precision Platformer" to the end of the title some months ago -- such subtitles make a title immeasurably more clunky, and I doubt there are many other games with that exact name that made such distinction necessary. Also, having "Highlander" in there makes it sound as if it might involve taking people's heads, and with them, their power. ;D Supported OSes are one of those things that are often not accurate for "coming soon" games. I wouldn't read too much into that particular "data" point until the game is purchasable (though, of course, GOG does seem to have long since moved Linux support to near the bottom of their priority list).

I don't understand what GOG has to do with a Linux version though. That's up to the developers.

Possibly the main point was the "precision platformer" to emphasise that it isn't an easy game, to make it less likely that people who will be put off by the first few seconds of the game (almost everyone, I suspect, unless the update is quite extensive) will try it expecting it to be something they might like (being a free game a lot of people probably won't read the description). I am surprised at the number of positive reviews it got. I like platformers but I agree with the review by TheHostage that it has promise but rarely fun as it is (or was, I'll give the update a try).

Pink Hour (wishlist)and Pink Heaven (wishlist) are free platformers that would be great to have on GOG. My first thought as well although I think that strong connection may just be a US thing. The movie wasn't my thing at all but I enjoyed the television dramas.

Perhaps, but my point was that that info -- dry descriptive phrases -- shouldn't be part of the title at all, but could instead be prominently placed in the description on on the product page. If people buy a game without reading any of the description, that's their problem. This game being free makes it even more puzzling -- even if someone just claims it without knowing what it is, they can't exactly get outraged that it isn't the kind of game they assumed, since they're not out any money, and the download size is rather small. I was mostly joking about that part. :) But the main TV series (and presumably its spinoff, though I don't know about any of the movies or the cartoon series) was a Canada/France co-production, and is presumably at least as well known in many other countries as in the U.S.
Even ignoring that entertainment franchise, I'd say the word highlander has much more specific associations in English than highland. The latter can mean any mountainous area/region of high plateaus, while the former -- when no context is given, and especially when capitalized -- almost always refers to a person or thing from the Scottish Highlands.