Some never left. Notoriously, some bigtech companies have an internal IRC network for tech discussions because that's one communication system that you can rely on to stay up even during catastrophic outages.
I've been using https://www.irccloud.com for years, it takes care of all these things and also has an app / push notifications. It's still IRC, but it takes care of a lot of the annoying parts of it that made it feel dated.
It's as centralized as everyone running their znc on Hetzner. It's mostly just a bouncer with a nice web interface (Like The Lounge) and a rudimentary app for people who want to pay someone to run this setup. You'd still connect to Libera and other networks and there's zero lock in.
I never worry about presence in any form of chat. I don't care if people are there when I message them; it's up to them to pick up my messages at their convenience.