I usually have time walking for Tirisfal Glades on, and found that the Brill Flightpath was gone from Undercity. Going there directly it says it’s not bound to any other path.
I left Time walking, and found the flight path showing, but apart from getting killed once from the green mists, there was no flight handler there.
I think there’s a bug that moved the flight path from time walking to non-time walking.
P.S. I’m running TomTom and Grail, Grail NPCs and Grail Quests (enUS) outdated add-ons, just in case that matters, however i doubt it’s an influence on game data.