This should be the jive.usercontainer.thread.enabled=false system property - Jive would have to set it to true. Currently one must select "In a place" when creating a discussion - with this setting enabled one can also choose to keep it private and add users. But one can post also into the whole community (only visible in the activity stream) instead of selecting a place.
PS: I just disabled "Structured Outcomes" and hope that nobody is missing it. This is also the new default setting for JiveX.