The phisher will not receive a valid token, though, because you sign something that contains the domain you are authenticating to.

The phisher can just pass on whatever you sign, and capture the token the server sends back.

Sure, you can probably come up with some non-HTTPS scheme that can address this, but I don't see any site actually doing this, so you're back to the unrealistic scenario.

No, because the phisher will get a token that is designated for, say, mircos0ft.com which microsoft.com will not accept. It is signed with the user's private key and the attacker cannot forge a signature without it.

I think we're talking past each other a bit here.

If I were trying to phish someone, I wouldn't attack the public key crypto part, so how domains come into play during authentication doesn't matter. I'd just grab the "unencrypted" session token at the end of the exchange.

Even if you somehow protected the session token (sounds dubious), there's still plenty a phisher could do, since it has full MITM capability.

A password manager is also not going to fill in the password on mircos0ft.com so is perfectly safe in this scenario. You need a MitM-style attack or a full on client compromise in both cases, which are vulnerable to session cookie exfiltration or just remote control of your session no matter the authentication method.

Doesn't work for FIDO-based tokens, they auth the site as well, so won't send anything to phishing site.

From my understanding, FIDO isn't MFA though (the authenticator may present its own local challenge, but I don't think the remote party can mandate it).

There's also the issue of how many sites actually use it, as well as how it handles the loss of or inability to access private keys etc. I generally see stuff like 'recovery keys' being a solution, but now you're just back to a password, just with extra steps.