If a ticket reference is given in the check-in comment, the ticket should be
updated automatically with a reference back to the check-in.

You end up with a nice (automatic) trace of all changes pertinent to the ticket
(if the developers have the discipline to always provide the ticket reference).

This is particularly appealing in fossil since it comes with the ticketing embedded.

In fact, many organizations adopt the rule - no check-ins without a ticket.
Perhaps this could be a fossil option.

Note: it is possible for a check-in to apply to multiple tickets