If the group is automatic, users are only deleted once they no longer fulfill the criteria (in the past it used to be possible to manually remove people form automatic groups, but this option is gone since at least a year).

On a positive note, with this month’s update, we will get an option to add a condition to an automatic group, that only Active users can be part of a group, so once you deactivate an account, the user can be automatically removed from a group.

Oh yes! That is great! That will help with all the people that are deactivated. Thanks!