After avoiding the death of his mother at the end of season 2 of The Flash, Barry Allen mainly resets time back to what it was supposed to be. One of the changes is that Julian Albert works with the Central City Police and Team Flash.
Julian came to Central City in search of the Philosopher’s Stone, which he thought might bring his sister back to life. While there, he came under the control of Savitar, the main antagonist of that season of The Flash.
His disappearance at the end of season 3 of The Flash was a surprise, as it was not announced until after the release of the first trailer for season 4 of The Flash.
Presumably, Julian could be somewhere in Prime-Earth, but there is no confirmation yet. Tom Felton has shown interest in returning to The Flash to reprise his role, but so far there has been no movement on that front.
It’s unclear why Julian was dropped from the show and has never really been addressed. Julian’s departure in the time jump between The Flash seasons 3 and 4 received a weak story explanation.
Even though she was a part of season 3, The Flash’s most varied season, her presence was one of the best-received parts of the season, leaving her disappearance from the show a bit disconcerting.