Outlook Monarch controls are available to help with the deployment of the new Outlook for Windows client in a mixture of Exchange settings and registry entries. You can block users from using the new client or adding consumer email accounts to Monarch. And best of all, you can disable the “try the new Outlook” toggle until you’re ready for people to plunge into the brave new world of the revamped Outlook for Windows.
https://office365itpros.com/2023/07/17/outlook-monarch-controls/
