Microsoft has implemented a new synchronization mechanism in Outlook ProPlus to deal more efficiently with shared folders. The new approach increases the limit from 500 to 5,000 folders and is a more elegant and precise solution. Users who manage other peoples’ mailboxes will appreciate the change after they install build 11629.20196 or later.
https://office365itpros.com/2019/06/06/outlook-increases-shared-folder-limit/
