keep visibility group for deactivated users
If you are deactivating a user and have a setting for his deals like "Owner's visibility group" his teammembers in the same visibilitiy group can not see his deals anymore. In many cases somebody has to take over urgently and a sales director doesn't want to distribute the deals manually. Technically, deactivated user should remain in their visibility group.
And by the way: You should be able to filter for their deals and they should stay in the same team. Otherwise the team statistics will not be correct.