The Thieves’ Cant feature lets you hide a message in a conversation, providing the person you are talking to also knows thieves’ cant:
During your rogue training you learned thieves’ cant, a secret mix of dialect, jargon, and code that allows you to hide messages in seemingly normal conversation. Only another creature that knows thieves’ cant understands such messages
Unfortunately, the official materials I am aware of are silent about what creatures actually know thieves’ cant. When our rogue tried to speak thieves’ cant in a group of bandits, I handwaved that one of the bandits was actually a former thief and could understand the message, but this was a DM's fiat.
RAW, who are supposed to know the cant? I've never seen the Thieves’ Cant feature in a monster's stats block, neither in a NPC description.