So you're expecting valve to put in the time to maintain compatibility with a platform that drops support for old APIs and refuses to adopt new mainstream ones?
Valve has clearly decided, that it's not worth the effort. Apple is hostile to developers and creates extra work because it wants all things to go through their systems.
And I agree. That apples actions make the correct business decision for valve to screw over a minority of users like you, is on valve, but also apple. That Valve needs nothing from apple to run on their systems is a patently stupid claim. Yes, they can make the changes necessary to get the client running a little better. But why would they? The whole gaming ecosystem is doomed if apples non-existent support for it fails to improve.
The difference on platforms like linux, is that valve has joined as a contributor for the whole ecosystem, when they need changes in the OS itself, they themselves can make them. And on windows the graphics pipeline, DirectX, is already industry standard, and the OS ALSO still supports all older versions, AND openGL and Vulcan, too.
Gaming on mac, is dead right now.