Back in 2016, Microsoft introduced the ability for co-editors of an Office to chat with one another. Users were using this feature to communicate with others immediately, for example, to ask a quick question or divide sections among the co-editors.
Microsoft today announced Office 365 customers that co-editor chat access inside Office apps across web, Windows and Mac will be retired over several weeks beginning mid-April. Microsoft is recommending users to use Microsoft Teams when users want to communicate with each others immediately.
It is important to note that the ability to attach a comment to a specific selection within a document is still available. This feature can be accessed through the Review tab on the ribbon. Comments added this way are saved with the document and can be replied to, marked as done or deleted.