I bet it wouldn’t be too hard to build a nice UI over a mailing list. You could make it as nice as Slack. Has this been done?

Spent some time on this a few years back and have a half-finished project for it. The thing that proved to be a huge roadblock was importing existing archives since many don't provide a good interface to it.

The https://lore.kernel.org site is actually fairly decent, but limited to Linux kernel stuff and some adjacent projects. Gmane was quite nice too, but now defunct (the web UI anyway).