You can't access the full filesystem from the steam runtime container

Ah, thank you. Was that a recent change? Because it has worked before, on the same system.

No it's not a recent change. This only applies to games run in proton. Native games should still be able to reach the full filesystem

Mh, this is strange. I have used it in "Dying Light 2" and "Cyberpunk 2077" with steam and "Diablo 3 & 4" with Lutris before, including distro info. All of them always required Proton.
I have just installed a native game for testing (7 days to die) and it shows no hud at all. :D

Sorry I should have been more clear. This case is only when using proton in steam. Lutris does not use the steam runtime

Lutris has been using the Steam Proton (and Proton-GE) version for some time now, which might explain why it also no longer works there. On a different note, is it possible to enable Steam to access information such as the kernel version so that it displays correctly in Mangohud when using Steam and Proton?

steam launches games inside container called steam runtime, so you can only use binaries that steam runtime offers.

you can enter that container and see what it offers for yourself like this:

sudo nsenter -t GAME_PROCESS_ID --mount --uts --ipc --net --pid --cgroup /bin/bash

@graygrotesque mangohud doesnt have kernel option, but you can use exec like this:

custom_text="Kernel version"
exec=uname -r