Time to dust off those old game CDs and test them with Wine 6. Don't forget to leave reports in WineHQ AppDB.

Since you mention games; I still don't fully understand the relation between wine and proton. It's mostly described as a wine fork optimized for games. But is it otherwise identical to wine wrt usage? Or is it integrated into steam? If it's standalone, should I even still bother trying games on wine? Does code from proton end up back in wine? Could I run other Windows apps with proton instead of wine and benefit, eg CAD software?

It is meant to be integrated into Steam, so building and using it without Steam might be a little less straightforward than pure Wine, but you can definitely do it. The launched script expects environment variables and other stuff from Steam, so it won't work without Steam. On the other hand, no other launching script is provided, but if you call the wine executable with the correct options and environment variables, it should just work.

On average I don't expect specific merits using Proton vs using vanilla Wine for non-game applications. As far as I know, Proton-specific patches are usually hacks targeted either generally at games or at specific titles. Actually, using Proton could be worse than vanilla Wine, because Proton is based on a past version of Wine which only gets updated every now and then (last Proton is based on Wine 5.13), so you are missing some development (on the other hand, sometimes developing features breaks stuff, so this could also go the other way around).

Ideally, we always want as many patches as possible to go into vanilla Wine instead of Proton or CrossOver; first because we love free software and second because maintaining forked versions is time-consuming like mad. However Wine rightly has strict code quality requirements, and sometimes developing a proper fix for some bug is either impossible or too long; those cases are handled with patches in Proton (perhaps shared with Wine Staging or CrossOver).

Disclosure: I work for CodeWeavers on Proton. Opinions my own.

Does proton use game specific forks?

I’ve always been curious, I stopped playing games shortly before steam on Linux, back then it was a mess of using various wine versions and configs on various games (or maybe I just sucked, I had far less luck getting stuff to work than others on Wine DB).

There are also some instances of game-specific behaviour: Proton detects at runtime which game is running and decides whether to enable or not some code. This is done for the most filthy hacks that are required to run something, but risk to spoil some other program. Search the code for instances of getenv("SteamAppId") or getenv("SteamGameId").