the insert comment option has disappeared from my right click context menu too. Running Excel Office 365. If I use one of the work arounds suggested above to insert a comment, the option then appears to edit or delete the comment.
But if the cell has no comment in it, I have to use 3 or 4 steps to insert it as opposed to one previously.... hoping this is a temporary bug because it stupid.
This thread started almost a year ago, so I guess it depends on your definition of temporary. To be fair, how long do those 3 or 4 steps actually take? Bigger picture, worse things happening around the world and all that.