IMHO a publicly/semi-publicly logged irc channel would do just as well, but that's even more oldschool.
As for Discourse's mockery of a mailing list mode, let's not even talk about it.
GitHub has email notifications for issues, but you can opt out of any particular discussion if it gets too pedantic or doesn't relate to you. This helps massively reduce inbox clutter.
The thing that bugs me about mailing lists the most is you get all the email, all the time, forever.
This is a total non-issue. Mailing lists support daily digests if you want that, and email clients support folders and filters if you want that instead. Nobody managing 5+ mailing list-based projects at once is dumping all of that into an unsorted inbox.
Because who doesn't like mucking around with their client's filtering?
Isn't there a decent mailing list package that hybridizes a GitHub "issue" type system with a traditional "email firehose" approach?
Because (your email client) is always going to be much, much faster and easier to navigate than "some dudes cute forum setup".
Replying to and managing conversations is much easier when you can do it with one or two keystrokes rather than mousey-mousing ten clicks all over the place (and oh their ad tracking js is stalling out again...)
I maintain that all web forums should have a mailing list interface so that you can use the forum without using the web at all ... but I suppose that breaks their revenue model ...
(Personally, I use GMANE+NNTP for mailing lists, and NNTP is probably better than plain (public!) mailing lists for most purposes, but unfortunately that ship has sailed.)
IRC is still an underused tool in my opinion. The ability to just talk about one issue in a Mail List and keep track of the communication is great. I can't think of another tool that manages communication as well as a mail list (Forums are just not as good in communication notifications like a mail list.)
P.S. I still hate Mail List and don't use them anymore but nothing does it as well right now.
IRC, like other chat and IM tools is synchronous. That is useful to get an issue solved quickly. But mailing list are asynchronous. People can think before answering and don't have to be around at the same time. Also threaded nature of mail makes it better to archive discussions and referencing them later. An IRC log is full of other noise and little structure.
If I want long-form messages delivered asynchronously, what other choices do I have (forums?) and why are they better than mailing lists? (everyone already has an email client.)
Or is it because you value the communication differently than the code? We've evolved distributed revision control to handle issues or geography, connectivity, and work styles effectively allowing you to be self-contained and then collaborate (push, pull, merge someone else's stuff) when you are ready to. Email is the only generally available method of communication that works the same way.