This rep achieve still shows in tab even when it is no longer available. Not fair to tease is with something that can no longer be attainable. Please remove.
It causes frustration for those of us who want to complete something that Blizz has neglected to remove.
No one in tech support, or any Support forum, can help with this.
It’s a developer choice, and no forum here goes to the developers’ attention except Bug Reports. And this is not a bug (it’s how the developers chose to implement it), so it doesn’t go into Bug Reports.
Oddly, your original post in the Achievements forum would function perfectly well as a request to the developers to change their design choices. Your post here was completely unnecessary.
Editing: look at that, a SFA very thoughtfully moved this thread from Tech Support into GD where it can do some good.
Try killing a Celestial on the Timeless Isle. Rinse and repeat ad infinitum. You’re welcome.
I just wanted to add to this but if you did some of the MoP legendary cloak questline then you can still gain rep with him outside of farming the Celestials on Timeless Isle. You just have to go to the Isle of Thunder and kill Mogu for some reputation.
Truthfully, I’m not entirely sure how far into the questline you had to be or exactly where to go for the Isle of Thunder mobs (been a minute since I did that reputation and I have a horrible memory,) but I did do it fairly recently so I would assume it still works. You can check the Wowhead page for the reputation to get more details.
I hope this helped haha, sorry for the lack of detail but I wanted to mention it anyways. 
Best of luck to you!
Lots of reputations aren’t obtainable anymore and may still be shown.
honestly IMO, they should NOT remove them from sight
those who stayed dedicated and played the content, deserve the badge of completion forever imo
not their/our fault SOME players just arent dedicated to getting the reps when they are live…
Don’t take my rep. I worked for that.
It is obtainable.
Hey, me too. I’m getting close to exalted killing him weekly with my hunter who parks in the grummle tent and only runs out to kill a celestial in order not to lose her dmf buff.
What engaging gameplay!
IF:
You STARTED that Black Prince line WAY back, in Pandaland, you can STILL get rep from those mogu stone trash kills on the Island outside (yeah, Thunder, as above mentions, it is just the outdoor mogu dudes around ruins and stuff.); My hunter happily found this out, after looking for a bow mog from them and then getting to exalted with Black Prince.
if you DIDN’T start it back then, it is killing a Celestial dude once a week/whatever, forever.
Go to Thunder isle and farm it.
I reached exalted from neutral last year.
I always wondered about that rep, because I missed out on MoP. I just never bothered to look it up either.