Slack has the advantage of maintaining message history server-side instead of client-side, so you can catch up on things you missed without running a client in a screen session (or a bouncer, or a logger, or...). This functionality is part of the core protocol.

Is there a similar standard for IRC? If not, then IRC is serving a different use case.

It's trivial to log IRC, but it is hard to integrate server-side log display and search with prebaked clients.