You can already subscribe to projects or single issues/PRs on github and reply via email to post comments.

I can understand not wanting to use GitHub/GitLab/etc. for various reasons. But I don't understand how usability vs mailing lists is one.

How is a set of 9+ mailing lists any better? It has significantly worse discovery and search tools unless you download all the archives. So you're creating a hurdle for people there already.

Then you have people use all kinds of custom formatting in their email clients, so consistent readability is out the window.

People will keep top-posting (TOFU), transforming the inconsistent styles in the process. Creating an unnecessarily complicated problem for your email client to "detect quotes", or you have to keep reminding people.

Enforcing structure of any kind in email lists seems so tedious. I'm not advocating for bugzilla style "file out these 20 nonsensical fields before you can report anything" but some minimal structure enforced by some tooling as opposed to manual moderation seems very helpful to me.