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.

Crashed client / bouncer should mean it's still in their local log. Bots can also deliver messages pretty commonly. Usually it's `.tell nick message` and next time that nick connects and posts they'll see it.

The difference between a forum and a chat is mostly UI and structure.

On a forum, every conversation is within a thread for example.

Chat is unthreaded by default, even if Slack allows threads. It also allows users to create channels and manage who's on those groups, while the forum equivalent is fixed up-front and only opt-in.

So really chat is a forum that's a bit more free-form.