@LeoMcA is also grappling with this issue on behalf of Mozilla. Would appreciate it if you could chime in with your perspective on this Leo.
I think the best solution to this is a combination of a better Watch workflow, and the addition of email tracking so that a post opened by email = a read post. (Alternatively, opt to not receive in-app notifications for messages you have received by mail).
Mailing list mode isn’t feasible on a large forum. That’s why you move to Discourse when your mailing list grows unsustainably large!
Past that tipping point, the superior workflow is to Watch the categories that you want to follow. In mailing list land (see Mozilla’s lists for example) the comparable workflow would be opt in to however many mailing lists you want/need to follow.
Strictly hypothetically speaking, maybe we’d be better off doing away with “mailing list mode” as this special different-from-the-rest mode, and replacing it with an easy way to “Watch all categories”, which can be granularly changed from there. This is probably in line with what @mcwumbly has in mind for “user saved filters”.