returned operator functions from multicast operators share, publish, publishReplay are now referentially transparent. Meaning if you take the result of calling publishReplay(3) and pass it to more than one observable's pipe method, it will behave the same in each case, rather than having a cumulative effect, which was a regression introduced sometime in version 6. If you required this broken behavior, there is a workaround posted here (#6410) (e2f2e51), closes /github.com/ReactiveX/rxjs/pull/6410#issuecomment-846087374#5411
Features
All subjects now have an observed property. This will allow users to check whether a subject has current subscribers without us allowing access to the observers array, which is going to be made private in future versions. (#6405) (f47425d)
groupBy: Support named arguments, support ObservableInputs for duration selector (#5679) (7a99397)
share: use another observable to control resets (#6169) (12c3716)
