MicrosoftUpdate, I've got my old comments back. In this thread user Bgwilson commented
To remove the modern comments feature try deleting the following two keys from the registry. You should make the usual backup first.
[-HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\ExperimentConfigs\Ecs\word]
[-HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\ExperimentConfigs\FirstSession\word]
After doing this (and disabling auto office updates to be sure it sticks) and rebooting (not sure if needed), my trusty old comments are back. No reverting to older versions needed.
Disclaimer, I have no idea whether this can break anything else, so create a Restore Point and/or export the keys before deleting them so you can merge them back if needed. Remember, messing around with the Registry Editor can break lots of things quite badly, do this at your own risk.
To find these keys, open the registry editor (WinKey-R then type regedit (enter)) then once in regedit hit CTRL-F and search for ExperimentConfigs. Again, first export the keys about to be deleted so you can revert the change if needed.
Hope this helps others. If it causes problems, please report back here.
Edit: this solution works but unfortunately doesn't stick on my system, hopefully others have better luck.
First I had Office set to not update inside Word > Account > Office updates, but it looks like the Windows app store ran an Office update anyway, and possibly related: modern comments came back. I removed the keys again, and disabled automatic updates in the windows app store. Today the modern comments are back again, as are the keys. Not sure what keeps regenerating those keys, word itself, or a separate service that I can disable. If anyone knows a better solution, please post, thanks!
