I used IRC for over 10 years, but after using Slack for 1 it just felt outdated to me.

Offline messages and simulatnous logins from multiple places are just two features I was missing in IRC and didn't even know till I had them, haha.

But yeah, IRC networks just worked as message broker and not as databases, so I never thought about it.

Slack isn't an IRC replacement. It's business trying to embrace, extend, and extinguish the IRC protocol so they can capture the business market that uses IRC and collect rent.

That's why I use Mattermost, which Slack Without the Suck™. It's free, fast, easy to use, visually appealing, supports PostgreSQL — what more could I ask for?

Well, a native Android client. But other than that, what more could I ask for?

Compatibility with a real protocol, IRC, instead of just a marginal bridge. Mattermost will still splinter communities.

Matterbridge connects Mattermost with IRC, XMPP, Slack, Discord, and Gitter: https://github.com/42wim/matterbridge