ITIL users used to be an easy query, because it used to be the case that we manually added every ServiceNow user with an assignment group to a separate assignment group called ITIL.
After running like that for a year and a half, it was clear that this was a lot of work and mindless maintenance to keep things going.
Instead, we modified every assignment to grant the ITIL role, and we decommissioned the ITIL assignment group.
This change was non-controversial, and freed up the ServiceNow administrators to do more valuable things with their time.
However, this broke reporting Venkat was using for generating a report for ITIL users.