No one ever said officially..... albeit we saw monk added at the time that durge was, and some of the gear you get works very well with the ways of shadow monk. Lorewise and story wise it works perfectly well, as shadow monks tend to be evil/ chaotic, which is durge's alignment.

Likewise, wizard, barbarian, cleric, Rogue, Warlock, Fighter are taken by other origins. No way durge is paladin, druid, or Ranger as those are usually reserved for good heroes, and Volo is a bard so that takes the class out of the equation. Only monk is left.