I'm no defender of mailing lists as a substitute for real bugtrackers (I'm fairly strongly against it, actually), but GitHub is also apt to drive people away more than its fans admit.
There's also the matter of its privacy controls being extremely lacking. It's another site that takes the position that you don't get a choice on everything being "social" now, so they'll index your activity on a central timeline whether you ask for it or not and broadcast it to anyone else who does ask for it, leaving you with no recourse except to opt-out entirely.
I agree that Github is not the solution the open source community is looking for, and feel the pain of the lacking privacy controls. Still, I feel that development on Github is much more accessible than that on a mailing list, and there's probably a solution which would be more accessible but still encourage people who aren't part of the mailing list culture to contribute.
For example, mrsh has its development on sr.ht[1], but accepts pull requests on Github or the mailing list for the project[2]. This seems like (almost) the best of both worlds to me, where people who want to use a mailing list can, and people who want to use a GUI can, though there is overhead for the maintainers to fuse together the results at the end. An alternative like Gitea or GitLab might be better for your mentioned privacy concerns while still lowering the barrier to entry that a mailing list workflow presents.
Design is not my passion, so I'm drastically underqualified to comment on whether any of these websites is an effective implementation of their ideas.
https://news.ycombinator.com/item?id=24378401
There's also the matter of its privacy controls being extremely lacking. It's another site that takes the position that you don't get a choice on everything being "social" now, so they'll index your activity on a central timeline whether you ask for it or not and broadcast it to anyone else who does ask for it, leaving you with no recourse except to opt-out entirely.