There are a substantial number of non-commercial FOSS games that aren't on Steam.

Many---though not all---of them are packaged in Linux distros.

Luanti, basically a FOSS Minecraft clone, is one example; that's in Debian.

A number of older games occur to me that aren't either in Linux distros or on Steam. They aren't necessarily the top FOSS games, but people may not be aware of them, if they rely on looking in distro packages or Steam to find things.

Linley Henzell, the guy who did Linley's Dungeon Crawl, which later became the well-known roguelike Dungeon Crawl: Stone Soup, had a couple of open-source games that he wrote way back. Overgod looks like it's been packaged and is in Debian. Lacewing is in Fedora, but not in Debian.

Video of Lacewing

Video of Overgod

There's a genre of games based on Paradroid. One of the first games I played on Linux was in this genre---NightHawk. It used to be in Red Hat Linux, but it looks like at some point, it fell out of the package repository, as it doesn't appear to be in present Fedora, and it's not in present Debian either today. Not on Steam, either.

I can't find any videos of anyone playing NightHawk on YouTube.

Abuse is a DOS-era action platformer, heavy on jumpscares, that was open-source and had some of the data files (the ones for the shareware version) put into the public domain. Could use the full game's datafiles too, if you got ahold of them. I think that that used to be packaged and in Red Hat Linux at one point, but it's not in Fedora or Debian these days, and it's not on Steam.

Video of Abuse

Surprisingly, while Empire---a very early game in the genre---is in Debian, xconq is not, and doesn't look like it's in Fedora either. Neither are on Steam. You may have played later hex-based strategy wargames in the genre, like Battle for Wesnoth.

Video of xconq

There is a very large collection of non-open-source-but-free text-based interactive fiction out there. These are games that are all or almost all text, where one writes instructions to the game in a somewhat-natural-language format and has the game respond. Zork is a (not-free, commercial, but you may be aware of it) early example of a game in the genre. Sites like IFWiki and IFDB catalog these. Many of the runtimes exist on Linux distros; gargoyle-free is a graphical frontend in Debian that can display games written for many different engines, and there are various terminal-based frontends, but the actual games are not generally packaged in Linux distros or on Steam. One notable exception---Anchorhead, a Lovecraftian game, was updated and had a commercial (and graphical) release on Steam a bit ago (which I cannot seem to get running on Proton under Linux, though given that it's Electron-based, maybe it's possible to run it natively using chromium...I'm not really long on Electron familiarity). I think that the reason that they haven't seen more popularity recently is partly because of the rise of the smartphone and touchscreens and onscreen keyboards. It's a pain to play them without a real keyboard, which many smartphone users don't have available. If I were going to recommend one particular game that I've enjoyed, maybe Babel, for the TADS interactive fiction engine.