Persistent on-server chat history that's identical across all devices is a major element for the non-technically-inclined.

Quality of Life reasons in my opinion. I think, for one, emoji reactions add a lot to a group chat. If there's no option to simply "like", "laugh" etc, people will write their reactions as text, and that quickly pushes the original message up, out of the screen maybe too. (Same problem with old style forums, vs a reddit-like interface).

But this is just UI candy, could be implemented in an IRC client too. From a protocol standpoint, what IRC really lacks is the consistent handling of chat history. I can imagine IRC handling this too; every registered user could always be "online" on the server, and therefore receiving all messages, and when their client actually connects, they would see the messages similarly how the IMAP works for email. But this is really reaching now, isn't it? Why not just use something that's similar in spirit, but actually updated to the moders internet, like the Matrix protocol?

I miss the practicality of IRC for quick async conversations.

Instead Slack & co add server side search and effectively turn company chat into a eternal meeting with random participants.