Kenny "Arvaanas" Hudson, head of QA Live Operations for World of Warcraft, tweeted this message late yesterday:

[PTR] 7.1.5 PTR is taking off, and 7.2.0 is coming to town.Realms will be down for our first 7.2 build deploy today! pic.twitter.com/OfyVJpzmWg

— Arvaanas (@Arvaanas) January 19, 2017

For the complete in-development PTR 7.2 patch notes, hit the official site.