It’s unfortunate, but people leave teams all of the time. I inherited a team that had some turnover, and needed to get the backlog in to shape.
The first thing I did was to remove all deactivated users from tickets. It turns out that Jira allows you search for all members of a specific team. Combining this with
assignee not in gives us the following:
project = "MYPROJ" AND assignee not in (membersOf(jira-software-users)) AND status = Open ORDER BY created DESC
I also found it useful to find tickets raised by those no longer with the company to be triaged and closed if outdated:
project = "MYPROJ" AND reporter not in (membersOf(jira-software-users)) AND status = Open ORDER BY created DESC
Removing outdated tickets that I couldn’t follow up on helped trim the backlog massively, and unassigning tickets from those that have left the team means that the backlog matches reality.