As Eric suggested in p. 2., this will likely break things. E. g., SDL_GAMECONTROLLER_IGNORE_DEVICES env var set by Steam is about 10k in size but it is probably needed for proper controllers operation.
You are bringing in XDG_DATA_DIRS yourselves as an example, so if this will happen to be the largest variable removing it will break things even if it is removed after first Wine process is started: it won't propagate to the child processes and that will break things, the same Steam if it is not started directly as the first process or, just e.g., opening URLs in external browser.
I am not sure if there is a good and universal way to safely shrink environment, so far addressing that on system setup looks like a saner choice. Maybe a test that Windows indeed doesn't accept >32k env size would be great (as it is not apparent that it is actually such a limit or it is just very uncommon to have big env on Windows).
FWIW we have a hack in Proton made for some game with the similar issue (removing some named env vars) but it is in effect for specific game only, doing that for every app would break things (and such hacks and app-specific workarounds are not accepted to upstream Wine).