Exactly - Master is always, perhaps, interesting - but 19.07 does need some more work upstream - much of this goes to the change on the ath9k target to ath79 and device tree. Work is getting done there, and some GL-Inet targets have fallen off, and rejoined.
Production releases - Gl-Inet does need to ship stable/reliable code - and 18.06 is a good place to be for them to integrate their additions.