It's not impossible, but it'd be a pain in the butt. If you occasionally use some avx-512 infrequently, no big deal (but also not a big deal to just not use it). But if you use it a lot, all of a sudden your core count shrinks; you might rather run on all cores with avx2. You might even prefer to run avx-512 for cores that can and avx2 for those that can't ... but you need to be able to gather information on what cores support what, and pin your threads so they don't move. If you pull in a library, who knows what they do... lots of libraries assume they can call cpuid at load time and adjust... but now you need that per-thread.

That seems like a lot of change for OS, application, etc. If you run commercial applications, maybe they don't update unless you pay them for an upgrade, and that's a pain, etc.