I've always wondered how feasible it would be if an indie developer (or maybe even a big studio) could just put out a Linux build of their game and just say "we're not going to support this outside of these very specific constraints - or even at all - we'll fix issues with our Linux build if we can reproduce those issues in the environment we developed the game in... but that's it."

Then the Linux community gets another game. Does it not work on K.I.S.S Linux running sowm as a window manager and an entire custom userspace? Probably not. Does it work using the latest Ubuntu version? Probably.

But the notion that developers have to support every possible Linux configuration out there just seems toxic to the Linux game development effort as a whole.

I suppose I can only speak for myself but I don't see that as an attractive option, and what you're describing is mostly what I did do for the game release I mentioned.

If I sell on Windows (I do) and Mac (I do) then I have to support a certain range of OS versions and ongoing OS releases - even if that means (for example) I have to figure out how to 'notarise' a Mac executable so that a user doesn't have a big scary Security Warning pop-up. Not ideal, but fine. The challenge with Linux is that I would have to communicate against expectations - that I would have to make it clear that when I 'support Linux' it looks different to the support for Windows or Mac. I do genuinely think that 99% of Linux people get this, it's just the 1% that's maybe less forgiving of different standards.

For me, just personally and selfishly, passing the buck to Photon or Wine is an easier sell for my business.

I wonder - what's wrong with supporting just the Steam runtime and that's it? Why deferring to Photon/Wine at all?

Do you mean just support SteamOS? The biggest problem is, as a few other commenters have mentioned, there's no further break-down in categories other than "SteamOS + Linux" (and the url is .../linux). So you can't tell the Steam Store "I only support distro x, y and not z". I can't hide it for those who I am not actually supporting, Steam will advertise it to them regardless.

When your game runs under Steam runtime, the real distribution is (almost) irrelevant - everything in your address space is supplied by the runtime, the things you get from the host system is the kernel/kernel modules and services you talk to via IPC (i.e. X11/Wayland, Pulseaudio).

It solves the problem of what version of what library is installed (if at all, maybe user removed it as "bloat") on the host system. You get known set of binaries that you can test against / coherent SDK target like with Windows or Mac.