New Mattermost mailing lists

Yes. Our top example is probably the Chef community, which was migrated off of a mailing list:

https://discourse.chef.io/

I know the Mozilla Community also has to cater to a lot of users who prefer the mailing list workflow.

We're also planning to make mailing list subscriptions more granular:

In addition to that we now make it really easy for users to email in private messages to a Discourse instance:

The gist of it is, you can tell users who'd prefer not to sign up to send an email to team@mattermost.org, and this email will be set up to create private messages sent to the @team group, which you can reply to from within Discourse. There's no tutorial for it yet but feel free to ping me if you'd like some help setting it up.