I just discovered this problem and this thread. I am a regular writer, an academic, and I rely on Word to flag passive voice. I cannot believe this is gone!
Yup it happened. The fix is "in progress". No projected date for the fix.
Till the real fix is released you have a couple of options. "Retrograde Upgrade" back to previous version or 3rd party grammar checking tools.
Please do add your vote and comment about displeasure with this "improvement" to the UserVoice site. MS actively monitors UserVoice postings
Bring back all of the old grammar checking rules -https://word.uservoice.com/forums/304924-word-for-windows-desktop-application/suggestions/10009743-bring-back-the-valuable-style-checking-tools-that
"UserVoice.com" - MS Suggestion box sites
You can use the Office or Windows 10 "feedback" (**)applets but then your suggestions just fall into a black hole. You get no feedback from MS about your feedback.
A better place to submit this type of feedback the "UserVoice" forum. You can submit suggestions like this to the forum either by "voting" and commenting on a similar existing suggestion or by creating a new suggestion for
others to vote and comment on.
This next link has a longer collection of MS UserVoice.com sites:
http://answers.microsoft.com/en-us/windows/wiki/windows_10-desktop/ms-suggestion-box-sites-at-uservoicecom/7397119c-6d48-49e6-8d94-7bd0a50d214f
Go to UserVoice, either through specific links I’ve provided below or create your own suggestions there. That is where MS is looking specifically for this type of user feedback to bring them back to reality from what-ever lala-land they are working
in when they make these "improvements". Comments (and votes) like these in UserVoice helped to convince MS to roll back the "phat QAT" buttons in 2016
and to at least partially undo the 2016 changes to the way the Pen input works.
Please do copy the comments you've made here and paste them into comments in UserVoice. If you make a new suggestion, copy the link to your suggestion and post it back on this question so other people with the
same problem can vote for it and comment on it. (You’ll get faster action that way.) Please get all of your friends and co-workers who use Office to vote for your issue!