Why information is kept about users deleted from site collections

You can tell if the users have been deleted from the site collection
by looking at the UserInfo table of the content database. If the value
of the tp_Deleted column is 0, then the user is active in the site
collection. Otherwise, they have been removed.

SharePoint keeps this information around because it needs it to link
to the items placed on the sites. If it removed the users completely,
the items would become orphaned.

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