Drools 5.2 introduced backward chaining and Drools 5.3 made it actually working. I've talked it over with Mark Proctor and he said that he would back-port the fixes to 5.2 for us. This is a bug to make sure we don't forget about it.
Yes, thats my understanding also talking with Mark Proctor.
Backward Chaining would be a TP feature in BRMS 5.2. We cannot call it TP if it is broken. Should be backported.
All backward chaining fixes have been back ported. This was non-trivial, as it was a large number of fixes over a period of time.
The fixes were indeed merged. Should there be any fallout, it will be dealt with in other bzs.