Is it a Valve problem, or a Microsoft one at that point?
Nevertheless, you could still download your game and remove the thin DRM layer (tools available to do so), and then play them on your Windows<10 machines.
Obligatory IANAL.
If Valve is offering supplemental support on top of what the developer is offering (eg: running Windows games on the Steam Deck via Proton), then its Valve’s obligation to keep things running, as the developer has never committed to keeping things running outside their supported platforms. That said, additional support from the developer (eg: Microsoft/343i for Halo, Respawn for Apex Legends) to get games running on other platforms is always appreciated.