I have checked the PTR just now and so far, these legendary items are in the transmogrification menu.
- Thunderfury, Blessed Blade of the Windseeker
- Shadowmourne
- Dragonwrath, Tarecgosa's Rest
- Val'anyr, Hammer of Ancient Kings
- Thori'dal, the Star's Fury
- Atiesh, Greatstaff of the Guardian (all 4 color variants, Green, Yellow, Blue an Red)
- Tiriosh, Nightmare of Ages and Golad, Twilight of Aspects (The cataclysm rogue daggers, if you forgot them)
But these Legendaries aren't in the transmogrification menu.
- Warglaives of Azzinoth (both weapons)
- Sulfuras, Hand of Ragnaros
- All Pandaria Legendary Cloaks
- All temporary Tempest Keep weapons
I wonder why the Warglaives are missing. They are still considered as swords. Is it because a Demonhunter can transmogrify them since Legion or is it simply an oversight on Blizzards account? And yes, they don't show up as Demonhunter as well and i don't have the achievement were i can transmogrify them on retail. It seems to me a oversight or they still think about it.
It would be a real downer if Blizzards disallows them and keeps them locked for Demonhunter. They are officially swords after all.
Sulfuras is a mystery to me. It looks like a oversight. There is no reason to not include it, if they already include Thunderfury. The one from Cataclysm is a updated version, but so is the hidden Thunderfury skin of Outlaw Rogues.
Same for the Pandaria Cloaks. They were unique in design and yet, they don't show up. I once again guess it's an oversight and they will be added later on.
That the Tempest Keep weapons are missing is pretty much self-explanatory. Every model exists as a rare/epic item.
I left out the Legion Legendaries because you can already transmogrificy them and all have already existing models.