SSH Using YubiKey 5 and ED25519 Algorithm
As far as I know, macOS 11.4 includes OpenSSH 8.1, which does not yet understand the new -sk key types. This feature was only added in OpenSSH 8.2.
("Security key" keypairs are a distinct type from "normal" Ed25519 keypairs, because U2F/FIDO keys cannot be used to sign arbitrary data – they only sign things that look like FIDO challenge messages – so the SSH client and the server must agree to format the SSH challenge in a different way than what's used for regular keypairs.)