I see this post is from a while ago. Was the desired list ever obtained? I found this KB article on creating custom admin views: https://kb.tableau.com/articles/howto/determine-when-users-are-signing-in-to-tableau-server-by-using-custom-admin-views?… It does not directly answer the above question, but I think it might assist with creating the desired view.
As for the time the Tableau Server retention, I believe by default, all user actions are captured and stored in the workgroup database: Collect Data with the Tableau Server Repository - Tableau
Hope this helps!
Unfortunately when a user is Deleted from the Server, their record is removed from the registry. if you have any historical backups of the Tableau Registry, you could compare the _users view from that to your current _users view. depending on your logging level and how long your logs are stored, you might also be able to check the postgresql logs to see when any query was made to remove a user from the registry. Checking teh logs is a purely hypothetical idea though, since I have never looked at the Postgresql logs to see what information is actually collected there. Now going forward, you could save a monthly snapshot of any of the fields that you need to report for deleted users and the ids in a local db. When you then have to create the report, just join the snapshot with the _users view and show the fields where the _users table is null.
Hope that Helps!