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.
Best Answer
The only monster in the MM that knows the cant is the Assassin (p. 343).
In the DMG about creating NPC-s we find a table on page 90 that lists knowledge of the cant as a possible NPC "talent". However that table is not intended to represent the populace of the world and should not mean that a random 5% of the people know this secret language. It rather reinforces that the DM can decide such a feature for an NPC as they wish. So it is "left up to the DM", which is as RAW as it gets for 5e in many cases.