I sometimes wonder why there aren't any chat-based apps/UIs for mailing lists? Think UIs in the style of Discord/Slack/Teams/etc but with email/mailinglist(s) as backend.

IIRC there was Delta.chat but no idea how they are doing? (And if they integrate with mailing lists/formatting etc)

Deltachat is still alive and well, you can see it evolve on their blog: https://delta.chat/en/blog

The thing is, deltachat really focuses on encrypted-first, and if possible encrypted-only communications in tight-knit groups: servers have no authority, they're merely relays. Mailing lists are not built for that, they're the central authority point where all moderation happens, and being forwarders they can't work with e2ee. In the current setting deltachat isn't built for mailing lists but group or 1-1 communication work very well.

Mailing lists are largely a different mode of communication than chat, as alluded to in parts of TFA. I’d go as far as to say that mailing lists are mostly incompatible with chat-like usage patterns. It’s a feature, not a bug. Chat programs and protocols like IRC have existed almost as long as mailing lists, but were never a replacement for them.

I think one of the reasons is that mailing list usage expects/imposes some level of reflection time and editing between sending a new reply. Imagine if every time someone hit enter in slack/teams/discord a new email showed up in a thread somewhere in someone's mail reader.