Can't help much but originally webauthn came from Fido2 and old Fido devices, like old yubikeys, which only supported U2F, were de facto compatible with webauthn (as in: webauthn was only an upgrade server side).

Now Google killed U2F in Chrome (and hence Chromium etc.) but you can migrate your webserver to use webauthn instead of U2F and your users' old U2F keys shall keep working.

For the "new" webauthn, called passkeys, which is a modified webauthn: I've got no clue.

It's not clear to me if old hardware security keys shall keep working or if we'll all be forced to use software keys protected by Google/Apple/Microsoft.