If you have an account in a Nextcloud instance and has added the application for its support (and I think that must also add a configuration but I didn’t check yet), you can use it there.
Mailcow also supports it, but I think only as 2FA and the other service I know is nonfree, so I don’t want to mention it.
Support is still the process but I think that some or most mainstream websites support it at some point.
solo keys looks interesting! Am I right that it could be used as a password, not like a 2FA only?
upd seems it more 2FA solution
If you have an account in a Nextcloud instance and has added the application for its support (and I think that must also add a configuration but I didn’t check yet), you can use it there.
Mailcow also supports it, but I think only as 2FA and the other service I know is nonfree, so I don’t want to mention it.
Support is still the process but I think that some or most mainstream websites support it at some point.