Return receipts are already a thing aren't they? But most people don't actually want to be tracked, so the correct thing to do there is not allow any external resources to be loaded from an email, and to make user tracking illegal/codify that stalking millions of people doesn't make it okay; it makes it millions of counts of stalking and harassment.

HTML already gracefully degrades. If you don't support a feature, you just ignore it and continue. It works just fine unless you're shooting for exactly matching some figma design on all clients, which is exactly what HTML is not supposed to do.