Currently it is common to do appointment scheduling manually using proprietary services like Doodle etc.
RFC 5545 defines an open standard for sharing free/busy information. It would be nice if Thunderbird would support publishing and retrieving free/busy information in a standards compliant format. Integrated group scheduling using existing internet standards will promote interoperability and reduce the dependency of Thunderbird users on proprietary services.
References
* https://icalendar.org/iCalendar-RFC-5545/3-6-4-free-busy-component.html