This change in functionality of how to insert a comment in Excel is all part of Microsoft Office’s new multi-user software or coauthoring capability that enables those with whom you share an Excel workbook file and have granted editing privileges to multiple users (known as guest contributors) to make changes in real time to its contents.
Notes and File Sharing features in Excel 2016 Update
Worksheet with all notes displayed.
Worksheet with all comments displayed.
Adding an @mention in a new comment.
Saving the Excel workbook to be shared and co-authored to OneDrive.
Sending a sharing link to a quest contributor.
Using threaded comments to question worksheet changes made by quest contributor