This is now fixed per:
https://github.com/discourse/discourse/commit/33d0a23d84a4d22f28b33a44df67dee1d14c35a0
Whispers are no longer an “experimental” feature.
There are a few notes though:
If you have an old system where you had “experimental whispers” enabled and were using them make sure you run:
./launcher enter app
rails c
Topic.reset_all_highest!
There is an edge case I will work on fixing where unread counts are higher than what they really are. In particular, if there is a whisper (or deleted post) between your old highest read post number and last visible post number on topic. I have an idea on how to fix but it is not a show stopper.