Lore-wise, none of the allied races took part in the War of Thorns, just stop with nonsense, please.
And it been mentioned many times, it is unknown how the Nightborne reacted to what have happened to the Teldrassil, as many Nightborne still have their relatives in the form of the Night elves.
It was Tyrande who refused them when they asked to reunite with their kin, they did not voluntarily joined the Horde, joining the Horde was the second option.
They were accepted back, but with the pact that they will mentor young Nightelves to the path of the Arcane, as it was during the Kaldorei Empire, but many simple minds still think the Night elf mages are Highborne.
It was obvious from the World quests, and from the interactions with Tyrande, she disliked the Nightborne before even entering the Suramar. If you interact with Tyrande in the gates of Suramar, Tyrande literally says to you, after helping me in Val’shara, I did not expected to see you helping to these mana addicts.
If the “OG” highborne would still exist, the Nightborne is what they would have become eventually, the constant seek of power, and addiction, eventually they would have modified themselves into something far greater than a normal Night elf.
The Nightborne consider themselves far greater than a Highborne by the way.
Moreover, please keep in mind, being a mage does not make you a Highborne, the Highborne aka Quel’dorei in Darnassian means the Child of a Noble birth, meaning as long as your elf was a noble it meant you are a Highborne. Even the Kaldorei themselves believed that the Quel’dorei passes by blood, and you cannot become one. If you could become a Highborne by just being a mage, then whole of Moon Guard would have been a Highborne, Illidan would have been a Highborne.
Your elf is noble, and lives in the city? Congratulations, you are a Quel’dorei.
