I have played WoW on and off for a long time and I was playing a lot during the promotion for the Warcraft movie. I used to transmog the replica staff of gul’dan a lot but it was when you would have the item you were trying to transmog instead of it being available all the time. My question is, can I have the staff restored to my account? I don’t know which character it was on back then and it is not available as a transmog.
First, have you looged all your characters to see whether any of them have it? An item is not available to be learned for transmog until the character that has it is logged in?
Next, do you have deleted characters that might have it? Because you can undelete them and log them in to try and find it.
There is an Item Restoration function here
https://us.support.blizzard.com/en/restoration
if you deleted it on a character, but if it was so long ago, I doubt it would still be available. (But would you have deleted the Staff if you liked it so much? seems unlikely)