If a user who previously had access to your Tableau server gets deleted, removed or disabled from the Active Directory. Then that user will not get removed automatically from the Tableau server via AD Sync operation.
The Tableau server admins would delete the user from Tableau server. The reason being, if the user had created any content, then they can be reassigned, and so it does not get automatically wiped out.
more info can be found here on Tableau site
https://help.tableau.com/current/server/en-us/groups_globalsync.htm
How Business Intelligence Works 