This seems a bit like missing the forest for the trees. Steam does not really let you know if the game is DRM-free, does not guarantee it will stay that way, and does not provide a reliable way to back these games up in a way where they could be used without the launcher.
So… GOG?
And Steam. You just have to be picky. Even GOG sometimes lets a game with it’s own DRM slip. https://www.pcgamingwiki.com/wiki/The_Big_List_of_DRM-Free_Games_on_Steam
This seems a bit like missing the forest for the trees. Steam does not really let you know if the game is DRM-free, does not guarantee it will stay that way, and does not provide a reliable way to back these games up in a way where they could be used without the launcher.