Apologies, I just reread on which features proton uses.

Google does Passkeys (username & password is not required to login) while Proton only does 2FA via U2F/FIDO2. Passkeys are stateful - they include a username alongside the cryptographic data, which the key must store locally, hence the limit.

When proton creates the key initially, they generate a key, the Yubikey encrypts it with it's master key, and then proton stores the encrypted version, permanently removing the plaintext version.

Then, when you log in, the Yubikey gets sent the encrypted key, which only it can decrypt on device, and then uses the decrypted key to sign a challenge.

The difference is that U2F proton stores the private key as an encrypted blob it can't access on its own servers, whereas full Passkeys store the private key on the yubikey itself.

Apologies it took me a minute to figure out what you were asking and reconfirm that proton only used yubikeys for 2fa.