I have completed all the other Allied race questlines and the orc heritage armor quest but the quest to obtain the mag’ahar orc allied race is not available in the orgrimmar embassy.
Comment on another post about this:
I’ve just run into this issue myself, went through everything to unlock the Zandalari last night, then thought I’d go ahead and do what was needed to unlock the Mag’har, but couldn’t find anything as of April 12, 2021. What I did see was a seemingly unrelated quest just generically named “Caverns of Time” which didn’t jump out at me as peculiar since I’ve done a lot of toon hopping & grinding rep in CoT instances lately. It wasn’t until I opened my quest log to see if I’d missed anything, and it was only there that I was given any indication that it was connected to the Mag’har.
See if that helps.