From what I read, HM AotC/CE were still available until the next actual tier was out (HFC if I remember right, correct me if I'm wrong), and HM/BRF were considered a single tier. EN, ToV and NH are considered a single tier as well (t19 and its precursors). AotC/CE:Helya are still available until Tomb of Sargeras, but Xavius ended today. They changed the text description of the Feats of Strength for Xavius in the 7.1.5 patch to say "before the Nighthold opens" instead of "before the release of the next tier of content" or whatever it was before.
Just seems odd. It's like an admission that EN wasn't tuned properly and too many people are getting AotC/CE for their liking.