I have noticed on WOTLK PTR that Varimathras is not in Undercity anymore as in Classic and Classic TBC.
That is a mistake for using 3.3.5 data to create WOTLK Classic. Varimathras were originally removed during patch 3.3.0. Before that, it was available, and using the phase system he is replaced once players completed the Battle for Undercity quest.
“Originally, after the questline he was not seen in-game again by the players who completed the quest (as part of the “phasing” system used in the Scarlet Enclave and areas of Northrend) but was visible for the players that didn’t complete the quest. In patch 3.3.0, Varimathras and the abominations that previously patrolled Undercity were permanently replaced by the Kor’kron Guard and their captain Bragor Bloodfist.”
He should still be available on Classic WOTLK launch until ICC is released.