Grapheneos didn’t support pass keys last year when I checked, so you couldn’t use them at all. There was some APIs broken/missing between the OS to browser comms so you couldn’t use 3rd party apps for pass keys, like proton or bit warden. I have been actively experimenting and adopting passkeys and didn’t want to revert. It sounds like there is support now though, so I will give it a try soon.
No worries, ya I looked into it about a year ago when I was seeing if I could make the switch, glad there is support now.