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.
Keys under FIDO2 or U2F can do that if the service supports it.
Edit:
I recommend you Solo Keys instead of Yubikeys which are free software and free hardware.
I believe NitroKeys are open source too? i don’t think a lot of sites fully support FIDO2 tho
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.