> There's really nothing to suggest this has been the case.

Yes there is, like for example, EAC and BattlEye implementing Linux support (without attestation) for the first time. It is true that not all games enable it, but idk, you're implying there is no reason to believe it, which is not true.

> I very much expect Valve to implement a hardware backed attestation and signed driver model for future hardware.

There's really nothing to suggest that this will be the case.