Details
Phorge does not accept ssh keys of type sk-ssh-ed25519@openssh.com or sk-ecdsa-sha2-nistp256@openssh.com, which are keys backed by a FIDO2 token. This is easily solvable by just adding them to src/applications/auth/sshkey/PhabricatorAuthSSHPublicKey.php. An even better solution could be querying the ssh server with ssh -Q key to get the list of supported keytypes.
Answers
Looks like the original list was added in November 2013, and the -Q feature was only being introduced.
Also, the original diff says "If we end up with more than like 5 of these we should probably make this a warning or something instead, the only goal is to prevent user error."...
Update: https://we.phorge.it/book/phorge/article/diffusion_hosting/ says we support ssh 6.2 or later, and -Q was added in 6.3, so that would need to be updated too.