Here is how I handled this.
Instead of just a boolean flag, I also had a DateDeactivated. In hindsight, that date field is all that is needed.
That way I still could see the caseworker for an old case, yet when I went to select a caseworker for a new case, only current ones were in the combo.
Code:
Perhaps you can adapt for your situation?
SELECT Lookups.ID, Lookups.Data
FROM Lookups
WHERE (((Lookups.DataType)='Email') AND ((nz([Lookups].[DeActiveDate],Date()))>=#8/1/2019#))
ORDER BY Lookups.Data;
I picked an arbitary date just before the first person left.
HTH
It is also a good few years since I implemented this.