I don't have Yubikey, but I have a Ledger device. Ledger is very safe for signing things with private key without this key leaving the device and a lot of people have it, because they need it for crypto, which is getting more and more popular each year. You can write a simple app for Ledger device. This device could be used for password recovery or even encrypted content recovery without the risk of exposing your Proton account to be linked with ordinary email which can be used to reset my password or to track to whom a given Proton address belongs (and every gov agency has access to this). Ledger is safe, private, offline and so far unhackable (and lots of people have it already). You can also recover your Ledger account by the seed words, which people store in safe private places or memorize. I don't know much about Yubi key (never heard of it), but it looks like it can be easily used once stolen. Ledger is pin protected and you can only try 3 times or it gets wiped. This would be great value if we could safely recover password without losing the emails.