Which you can do pretty much anywhere already? I'm at least not aware of any major service not allowing you to register multiple SSH public keys.

With the obvious exception being, for whatever reason, AWS that only allows specifying ONE SINGLE KEYPAIR when creating new EC2 VMs. Whyyyyyyyyyyyyyy?!

When I was checking out their git hosting service (AWS CodeCommit), they only allowed for rsa keys (and not ed25519, for example.)