WebAuthn is currently supported only on Apple ports.
Just as a point of reference, Linux Firefox supports FIDO2/WebAuthn authenticators over USB as of version 114.0 (https://www.mozilla.org/en-US/firefox/114.0/releasenotes/).
Will this be implemented sometime in the near future? This blocks my work login flow substantially (both with email w/ Thunderbird and with vpn login via networkmanager-openconnect)
It's part of the GNOME STF project, so maybe, but it depends on prioritization. Next step is to solve https://github.com/flatpak/xdg-desktop-portal/issues/989. I won't accept new static permissions into Epiphany and other app developers shouldn't either, and adding functionality into WebKit that doesn't work under flatpak is not OK, so a portal is surely the next step.