This is a huge PITA for mobile 3D game developers. Google would announce a shiny new Vulkan feature and I'd set a calendar event for 4 years in the future to remind me to check if 75% of the user base had it available yet.

How does the equivalent work for PCs? Is the ecosystem just more mature, so that there aren't that many shiny new features? Do GPU vendors ship polyfills for older hardware?

Yes and yes (to a point), but the bigger thing is that the entire update doesn't trickle down through every layer of vendor involved in your "device". If you're a typical PC gaming enthusiast, you probably get Windows updates directly from Microsoft and GPU driver updates directly from Nvidia.

The Android model would be more like Nvidia giving a GPU driver update to EVGA, who has to adapt it for your specific card, and then give that to Asus, who has to integrate it into their Windows updates for your specific motherboard model.

Windows DirectX is well documented.

DirectX11 hardware does X/Y/Z. When people buy GPUs, they can see which DirectX version it supports, be it DX9, DX10, DX11, or DX12.

This is another reason to support GNU/Linux phones (Librem 5 and Pinephone) based on free and open source drivers, owned by the user, not by Google or hardware manufacturer.