SuperTux is a classic side scrolling platformer, very old game at this point. The code is Open Source (or if you prefer the term "Libre").

I just played it last year a bit on my Steam Deck and remember trying the game out years ago on my old laptop and before that, it was one of the first Linux games I ever tried in early 2000s from Live CDs. I digress. :D

This game is also available on Steam, but they last updated it there 4 years ago and this new update is not included: https://store.steampowered.com/app/1572920/SuperTux/ Presumably because the new version is still a beta. You can download the new updated version directly from Github: https://github.com/SuperTux/supertux/releases/tag/v0.7.0-beta.1