Mobile devices matter because they cannot maintain persistent connections. This by design necessitates a different architecture. AIM and IRC can have a single machine hold open a TCP connection and send and receive messages. A mobile device's modem wakes up briefly, performs some network chatter, then goes back to sleep. It's a fundamentally different requirement on the way the system is designed. IRC has evolved (through add-ons, not in the protocol itself) to shim this by using bouncers or having some sort of push notification based proxy for messages.
The other question is, how reliable was AOL? I have no idea how many outages they had, how long those outages lasted, nor what the impact was. Given the service was meant to be used as entertainment I suspect their SLA was one of best effort rather than the measured kind that Slack is leaning into (wrt using multiple CSes and using consistent hashing to pick which CS to send a message to, so architecture can be scaled as needed.)
I don't work for Slack but I work on high scale systems for a big company for a living. I'm pretty aware of the limitations of networks nowadays and the differences between systems in the dial-up era from now. Just the fact that Slack accommodates devices with push based semantics (mobile devices) and pull based semantics (anything that can open a persistent connection) is itself a large difference between systems of old and systems today.
Btw the TOC and Oscar protocols that AOL used have been reverse engineered and documented online so you can see for yourself at least at a protocol level what's different.