@BérangèreL and @ERTCZisty I checked a few accounts and also can’t access the property via settings or any ticket records, I thought maybe it would be accessible via a custom report (cross object with contacts, tickets, conversations), but that also yielded no results:
I also couldn’t find it on the list of associations for tickets:
My only guess is that it could be a reserved property - one that HubSpot has identified as a system property that will be needed for future functionality not currently available.