I would not expect/want that behavior from a chat app. If I was having a conversation with 3 people, and then a 4th person walked up and joined the conversation, I expect them to not be privy to anything discussed prior to them joining.
A “channel” that shows all participants the entire history seems more like a private forum thread than a “chat”, and should probably be distinguished separately.
Whatever you want to call it, if we're having a conversation, say, planning a birthday party for Bob, and we forgot to invite Alice to the conversation, through the magic of computers, instead of us having to start the conversation from the beginning again when every new person joins, they can just read the scroll back.
If you're the type of person having conversations about people that you wouldn't want them to hear, maybe you shouldn't be having them?
I understand the pros and cons, I just think there should be a clear demarcation of which is which. I can see a chat app deciding to keep this feature out if they think it would make the app too confusing for its audience.
A “channel” that shows all participants the entire history seems more like a private forum thread than a “chat”, and should probably be distinguished separately.