> Assuming, of course, that the platforms don't somehow intercept the passkey requests and refuse to allow KeepassXC to do its job.
My understanding is the ability to do that is built directly into the spec with the attestation feature. The only thing that might slow it down is Apple choosing to not implement it and zero out their device string. Others can piggy back on that to protect themselves behind Apple's skirt, at least until Apple changes their stance anyway.
Platforms of course could just not allow Apple passkeys and only allow Apple users to use other 2FA options as well. Rest assured that small players like KeepassXC will be the first ones to have their passkeys blocked or not supported.
The whole thing is a trap IMO.