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, ther

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.