See discussion from 3 days ago (69 comments): https://news.ycombinator.com/item?id=12576116

TL;DR: It is not in mainline, but it does not need a patch anymore. You need to bring your own conflict resolution logic.

Or design the application so that there are no conflicts (e.g. modifying different subsets of users on different nodes).

Which is a form of conflict resolution. It requires the application to be aware of the datastore.

I wonder if, since BDR is just a plugin now, a plugin that used strong consistency guarantees could be built using the same changes that were required for BDR.

BDR has last-updated-wins builtin and conflict handlers that can be called if that's not what you need.

What is BDR?

Bi-directional replication