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.