I was asking about mailing lists, but even so, people who are posting interesting newsletters are also very welcome.

BTW, thank you everyone who posted (or is going to post) an answer. There are many great suggestions!

I know you asked specifically about mailing lists, then most answers, including mine below, are more newsletters, but if you want a mailing-list type feel, there are also Patreon, Reddit, individual blogs, and other communities that communicate in much the same way and have the same sort of interesting mailing-list feel, with similar volumes. Not sure how much you'd considered those, but a lot of the Reddits I'm in would have been mailing lists/usenet topics pre-Reddit,

Even some public Slack and Discord servers have the same mailing-list feel. eg. I'm in the monitoring.love Slack with a monitoring/SRE focus, and the r9y.dev Discord which is Reliability/SRE/Monitoring focused, and both have the same mailing list type feel.

Consider the Handmade conference newsletter [0]. We're an indie conference focused on low-level programming (nuts and bolts kind of thing) and we occasionally send interesting emails on these topics. Of course we also announce ticket sales and all that, but sparingly :)

[0] https://handmade-seattle.com/newsletter

We've already had so many newsletter Ask HNs, I'd really like to see more about mailing lists today.

Alas, people are already posting blogs in this thread (so they haven't fallen for the newsletter/mailing list confusion, but are just posting whatever they'd like to push).

Indeed, that's mostly why I was asking. Mailing lists would be a bit more novel topic.