It would also be nice to have a way to quickly de-register a lost key. Having a back-up is great, but it's still a pain to go de-register the thing on each individual provider's platform. And if I miss one provider it could easily be game over. I'm not a huge fan of centralization but a single registry for dead keys would make a lot of sense for the different providers to cooperate on.
Most of the places where I've been able to use my Yubikeys (FIDO2) also offer a very clear list of enrolled/registered keys, along with either a hash or serial number of the key. It makes revoking a key pretty easy in my experience.
I'm less keen on the idea of centralized repo for dead keys - each key is unique to the site in question, so there will be no need to share the existence of that revoked key's identifer with other providers - the data is meaningless to anyone else.