This has been announced some time ago already (July 2016); Clutter is going to be deprecated and eventually removed and replaced by Focused Inbox.
Now, we have an official end of life date for Clutter: this will be January 31, 2020.
So you still have some time to communicate to your end-users, update your support team and training documentation.
You may also have to deployed Outlook build 16.0.8730 Version 1711 or greater to ensure you are getting Focused Inbox on your Outlook client.
If you are using Outlook 2016 MSI or earlier, you will have to move either to Office Click to Run or deploy the coming Outlook 2019. If you stay with these ‘unsupported’ version of Outlook for Focused Inbox after January 2020, messages will stop being cluttered and will be delivered to the Inbox as usual.