Earlier quoted context omitted.
Valve problem. Selling merchandise you know/stated the customer can't use is negligent at best, malicious at worst. Obligatory IANAL.
Seems like the obligation falls on developers and studios. It’s Capcom’s responsibility to keep Street Fighter X Tekken working because Capcom is the seller. Fortunately the community usually warns buyers about abandonware and you can always get a refund.
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.