Comment by rdmuser

Steam has this handy curator: https://store.steampowered.com/curator/38475471-Libre-Open-S...

Itch has this filter: https://itch.io/games/tag-open-source

As a bonus here are some random non abandoned lists I found with quick search:

https://en.wikipedia.org/wiki/List_of_open-source_video_game...

https://github.com/michelpereira/awesome-open-source-games

https://github.com/bobeff/open-source-games