Another factor is that Mac software was never able to rely on the availablility of AVX instructions anyway. macOS Catalina was officially supported on some systems with processors as old as Ivy Bridge, which lacked AVX support.
I thought it was up to the software (for example, a video encoder like Handbrake) to detect whether a processor supported a set of instructions, and then try to use them?
That's how it's supposed to work, yes. And any software that does that should probably work fine on Apple Silicon, by falling back to the non-AVX path under emulation.
I've seen some poorly behaved Mac software which used AVX instructions without testing for availability. It crashes if run on some older Macs, and it won't work at all on Apple Silicon.
