This feature is now in general availability as of version 4.3.0
The caveats being
You can only create/edit the TOTP on the mobile app, not in a browser
Once you add TOTP to a credential, you can no longer edit it using the browser, all edits must be done in the mobile app
Since you can’t edit the description field on the mobile app, once you add a TOTP to a credential, you can no longer edit the description on any client.