Regarding the statements that you need a Yubikey (for the FIDO part of passkey, given application support?) on GNU/Linux, at least Solo 2, Nitrokey 3, and Onlyk

Regarding the statements that you need a Yubikey (for the FIDO part of passkey, given application support?) on GNU/Linux, at least Solo 2, Nitrokey 3, and Onlykey hardware keys have free firmware. I can't vouch for them, but maybe also see https://github.com/psanford/tpm-fido https://github.com/psanford/tpm-fido and https://github.com/bulwarkid/virtual-fido https://github.com/bulwarkid/virtual-fido