Tanaan came after the “no flight ever” debacle and given the public’s general response to that, it is easy to see why the devs (who didn’t put much into post-leveling WoD) weren’t about to test the waters with a no flight end zone. Hence, Tannan was also the last content that went into WoD.
Legion unlocked flight and then a few short months later, issued a no-fly progression patch (unlike the totally optional Pandaria Isles).
I fully expect the same for BfA whose post-flight unlock patch will be an underwater area.