Old Yubikeys only support U2F, and there's a Yubico FIDO2 key. Browser support isn't there yet, I've been trying to write a Django library for it but no browser will support the complete FIDO2 flow as far as I know.
I mean, certificationally, sure. But what prevents a website from trusting you to input your identifier (user name or e-mail address) and then accepting a U2F signed blob as your only credential?
(I would suggest that a single physical device that takes a PIN before it does some signing is still 2FA even if there is only one _communicated_ credential, but I appreciate we need better terminology for this. Other versions of this to consider: if you username + password + Duo into your SSO portal and then sign into a service with SAML, is that not 2FA? If it isn't, does using a session cookie prevent something from being 2FA? For the latter, I'd say obviously not :-) I think the "who can impersonate you" is a good line in the sand, since in the former the SSO system holds full authority in most cases.)
Technically U2F can be used to design passwordless scheme too (returning a big array of all key handles known to a service) but FIDO2 probably has some optimizations in this area.