The mailing listS (plural, there are MANY of these, in many languages and countries/regions) are one of the ways that people EXTERNAL to the OSM project can both gain some insight (by reading) and offer some help or ask for specific assistance (by posting).
Were it not for our mailing lists a decade ago, I would not have been contacted by a “national route architect” who has, since, collaboratively with me and many other OSM contributors, managed to develop over the last decade the USA’s national bicycle networks in a sane, well-described, officially designated, wiki-documented, some might even say exemplary fashion. This is but one example of this, I personally both have others and know of others by others.
My point (and yes, this did happen a decade ago, and our “communications” DO evolve), is that there must be and remain “more open” methods, especially for those who are not Contributors to the project. (Think “lurkers” and those who “read the news, but don’t make the news”). Because of how open email is in the “Internet Standards” world, it is hard to replicate the wide reach that mailing lists have. Yes, I realize that traffic diminishes as other channels (like this one) become more popular, and “the network effect” of mailing lists “going quieter” makes them much less effective (with each and every person who stops using them), I still don’t think they are “dead yet.”
I salute all the efforts at consolidating the old help system and this new Discourse: it’s amazing for whom it is for and what it is able to do. But we do have other users, other audiences and other “reasons” for these communications. (Some, like Slack, are more chit-chat and interactive-oriented, some are more “for the record” like the way we use wiki to document tags and many other features of our project). And Discussions on wiki pages are not a “hassle,” they are a vital, integral part of how this project was, and IS, built.
SOME mail-lists can / should be “sunsetted” for sheer lack of traffic. But those lists are the place(s) to discuss that. This is not about being resistant to change, it is about recognizing the long histories, wide access and near-universal standardizations making practicality, compatibility and outreach good reasons to continue them.