I would like to confirm the seemingly universal disappointment I can read here.
I am also utterly disappointed by the lack of internal user testing processes that could have highlighted the controversial nature of this loss of existing functionalities and lack of thoughtful, deliberate process before making important changes such as this. At the least, this change should have been optional.
This is a serious change in many users' business process, especially for small businesses who have no power to impose separate document negotiating or commenting/discussion tools for documents.
This also shakes my confidence in the Microsoft Word (for Windows) applications, which I've been using a lot since 1989. Unfortunately, it is exactly the commenting functionality that has the biggest problem in compatibility with alternatives to Microsoft Word. But at least this one is a good excuse to dump Office 365 and buy 2016 again as long as that is supported, and try to find a reliable solution in the longer term.