It seems that you can no longer register a FIDO2 compliant hardware token, like a Yubikey, as 2-factor-authentication method on Google. It can now only be registered as a Passkey. If you have previously registered it as a 2fa hardware key, it still works: it’s just that the registration interface is gone.
I suppose it’s the natural progression of things, but it is a bit annoying if you’re not ready for Passkeys yet.
Luckily, my co-worker worked out you can actually still register it if you use the direct link https://myaccount.google.com/signinoptions/two-step-verification?flow=sk&opendialog=addsk.
Those that have multiple accounts can add the standard /u/1/, /u/2/, etc. to the end of google.com/. Thanks Jakub!