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.