I searched on Bugzilla and didn't find a bug for tracking the progress on implementing the "cross-wiki Flow topics" feature mentioned by Quiddity on this edit:
https://www.mediawiki.org/w/index.php?title=Flow/Pages&diff=prev&oldid=945604
So, maybe it doesn't exist yet or just needs better keywords for search :-)
For now, I'm creating this bug. Feel free to mark it as a duplicate of an existing bug for this.
Version: unspecified
Severity: normal