When a user is deactivated or when they choose to remove their profile from your app or website, there is work needed to clean up your database, and remove all references to this user in your app.
Having inactive users along with active users is clutter, and would only make things more complicated for you. And as you accumulate deactivated users over time, it becomes a bigger chore to clean them up manually. Which is why, when a user is deactivated, it would be useful to just remove the user from everything.
If you wish to prune your subscriber base so it doesn't get stale, then you can use automation to delete users automatically based on the rules you choose.
For instance, if a user clicks on a 'Delete Profile' button in their accounts page, you can automatically delete them from your database.
To do this, go to the 'Automation' page, select the trigger that says 'Clicked a trigger link', and enter the URL of the link that your user has to click in order to get themselves deleted.
Click 'Save' and the automation rule will be saved. From now on, anyone who triggers that particular link will get their profile deleted from the database.
Note that this is a permanent step, and users once deleted cannot be retrieved again.