A very klunky, partial, manual work-around is to use an automation workflow to alter the “Ticket name” property to include the ticket ID in the ticket name. This’ll place the ticket ID in the ticket’s visible name. Unfortunately, the originating email won’t be altered, but when you reply, you can cut’n’paste the ticket’s name into the subject line. All subsequent emails should then contain the ticket ID in the subject.
Would be much nicer if this was automatic though and was able to handle associating subject line ticket IDs with existing tickets.