But now you have to run a ZNC server as well - it's a lot more work for a very simple feature.

It's not "very simple" - it's just that with Slack, someone else is running ZNC for you.

Chat apps don't magically "just work" - either you run your own infrastructure, or use someone else's.

Even with IRC you are still using someone else' infrastructure -- unless you run your own IRC network from scratch of course.

If you ever want to feel like you're bashing your head against a brick wall over and over, set up an IRC server with channel/nick services.

WeeChat + relay clients can do it as well, you only run weechat in a tmux/screen somwhere and connect to it with a relay client (Glowing Bear or WeeChat-Android). Works nicely, with synced state and arbitrary history scrollback

I honestly just run a raspberry pi that has Weechat running all the time in a tmux session. So I just SSH whenever I need from wherever, pretty much (with obvious security settings configured).

It works really well and is a cheap solution.

ZNC is great. I run it on a Pi so I don't even really have to worry about reboots or anything on another machine.