Microsoft does it. It's hidden behind one click in the UX, but if you click "Sign in options" on the screen where it asks you for a username to sign into e.g. Outlook, one of the options is to sign in with a WebAuthn credential. Here, Microsoft invokes the WebAuthN API in "discoverable credential" mode (with empty allowCredentials) - if you have multiple online accounts connected to your Windows account, Windows will ask you which of the accounts you want to use and then provide both userid and assertion for that account.
