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).

It hasn't been done, because it's impossible. You might get the first 80%, maybe even the first 90% if you had enough money to hire top designers for several years, but you'll never reach Slack level polish.