I'm not familiar with this particular use of "being on", but it appears to me to be a small extension of a use that is familiar to me:
Are we still on for 3:00?
Which is informal and abbreviated of:
Are we still planning to
X at 3:00?
X is most likely some kind of meeting, which in turn is most likely an informal meal. The question is most likely being asked because an obligation may preempt
This usage is in reference to a specific social event, while the headline in question appears to refer to an ongoing social relationship. Nevertheless, it feels natural to me that "John and Jane are still on" indicates that doubts regarding the continuation of some relationship between John and Jane have been resolved.