it won't propagate to the child processes and that will break things [...] e.g., opening URLs in external browser
Only the Windows apps won't see these variables, but on the Unix side, all the envars are still there.
SDL_GAMECONTROLLER_IGNORE_DEVICES env var set by Steam is about 10k in size but it is probably needed for proper controllers operation
AFAIK, it's Proton/Wine itself that uses this envar, not the Windows app itself, so nothing should be broken even if it's "removed".