IDK if this just went under people’s radar, but ever since 11.1, they added a new whelpling NPC (next to the MoP Timewarp one) who sells many MOP tier set lookalikes appearances that are not class restricted - including some appearances that once were exclusive, like the Priest dragon horns.
Some appearances/color variations weren’t available before, like some tier chest pieces with cropped skirts.
Maybe Wowhead announced it, but I’m keeping up with the news and didn’t see a mention of it.
Thank you! I didn’t notice that little guy before. He sells some stuff from Panda Remix that we may have missed. He had a few things I didn’t have yet!
Also, for others looking for this whelp, his name is Nostwin and he flies around back and forth behind the ghost of Shaohao. His flying is rather irritating while you’re trying to buy from him as once you get to page 7 of his goods… he’s taken off again and closes your purchase window. sigh Or maybe I’m just slow.
Oh, didn’t realize those were from the Remix! Wasn’t subbed back then.
If you were diligent collecting collectibles in Remix, odds are good you’ll have all the transmog appearances (at least armor ones). But an addon like All the Things will still tell you the items Nostwin sells are distinct from what was available within Remix, which might be bad for your OCD.
The appearances are identical to what was offered in Remix, so if all you want is complete coverage in your Appearances collection, you may not have to buy anything or everything.
All the things can list old appearances from the Trading Post that you didn’t get? 
I use Better Wardrobe to see some missing appearances that are not in the regular tabs, but it never shows stuff like that. I’d love to see ingame what I missed from the months I wasn’t subbed, since they can eventually reappear.
ATT shows whether you know an appearance but also indicates whether you have learned all, some, or just one source of the appearance.