Developers don't care, they target Windows as usual, and let Valve do the needful.

Some game developers do test under Proton. Saying that because I'm in the discord server(s) for a few, and they mention stuff they're working on/support/etc over time.

Likewise, some do write GNU/Linux games, yet that wasn't in a number relevant enough to keep the original SteamOS going as businesses, until translation of Windows APIs was added.

Sure, but I think the point is that that doesn't matter anymore. Not sure what you meant by restating this, seems irrelevant.

Yeah that part is lovely. Having steam games “just work” for the most part is absolutely phenomenal.