The real problem was when Slack added message reactions. Those weren't visible at all in IRC. They absolutely could have been surfaced, like how Apple sends reactions in SMS chats. In a business context that means missing critical acknowledgements of messages and generally forced our IRC bridge users to switch.

It was a pretty user-hostile way to reduce the number IRC bridge users (to then justify killing it), especially given it took them many years after to get the Electron client to the point where it wasn't a laptop-killer.