I just now realised I get this in my dmesg when trying to launch the game:
[30804.632451] umip: dirt2.exe[135671] ip:4a5e87 sp:141c474: SIDT instruction cannot be used by applications.
[30804.632453] umip: dirt2.exe[135671] ip:4a5e87 sp:141c474: For now, expensive software emulation returns the result.
[30804.632454] umip: dirt2.exe[135671] ip:4a5e8d sp:141c474: SLDT instruction cannot be used by applications.
You'll need a kernel built with https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b91e7089ae70d2f7c81a4456e5b78fef498663d9 to deal with the SLDT usage. (https://github.com/ValveSoftware/Proton/issues/3975#issuecomment-703177796)
Updated to 5.10.0-rc1-1-tkg-pds today, game works perfectly aside from the 5.13-1 gamepad hotplugging issues. Like any other xlive games it needs to be patched out, but I am not sure it's allowed to be linked here (even though the one I used is open source on github).

Compatibility Report
System Information
I confirm:
steam-12840.log
Symptoms
Says "playing" for a few seconds then quits, with or without xliveless patch.
Reproduction
Install DiRT 2 and try play it.