When I tried it with POSTMAN, it said NOT IN CHANNEL and gave me an error.
It is true that a bot that is not basically participating in a channel (treated no differently than a user) cannot get conversation contents of the channel, which is strange, so it may not be possible.
However, as mentioned in another Stack Over Flow question (Can a slack bot join automatically a channel?), there was a way to post a message without joining the channel. In the same way, is there a way to use conversation.history for that channel without joining it?