KeePassXC added support for passkeys in 2.7.7, released a week ago:

https://keepassxc.org/blog/2024-03-10-2.7.7-released/

This post prompted me to give it a try at webauthn.io.

Passkey support is disabled by default, so you have to enable it in the settings for the browser extension. After that, the UI seems reasonable enough. It creates an entry with a "Passkey" tag, where the Password field is empty, and the credentials are stored as additional attributes on the Advanced tab.

One thing I'm not sure about: although the extension has a setting to fall back to the browser's own passkey support, when I dismissed the KeePassXC prompt, I did not get a prompt from Firefox.