how to delete an employee? I want to do this in development enviroment. The system says that particular employee was a supervisor of someone in the past (!). And that's true, there is a tab with supervisor's change history but I don't see any possibility to delete history entries from this tab. So could it be, that this employee record stays forever in the system?

It stays forever in the system.

just mark them as inactive. You can edit the record to remove/change any information you don’t want to retain.