Take an edit, and suppress its edit summary using an account with suppression right (i.e. Oversight).
Now, using another account that has revision deletion rights (i.e. Sysop) try to change the visibility of the username for that edit. You will get a permission error.
You do not have permission to review and restore hidden revisions, for the following reason: The action you have requested is limited to users in the group: Oversighters.
While the sysop should not be able to see or change the visibility of the edit summary, they should still be able to see and change visibility of the revision text text or the username.