Seems to me a plugin would be the perfect solution. For those users that want to receive a newsletter or general announcements, this gives them the opportunity to opt in.Those not interested wouldn’t be required to opt-in; they visit the site when they want to be updated. Simple as that.
I think there are plenty of legitimate use cases for such a plugin (or inherent functionality). For example, membership in the community might include a free series of educational emails or an online journal subscription. The Discourse forum might be associated with a podcast, and the newsletter would be sent whenever a new episode is published. Etc.
A user might want and welcome this type of communication; even prefer it.
As long as it’s made transparent what they are actually signing up for, and there’s the mechanism (provided by the email newsletter service) to opt-out, then I don’t see the problem. In fact, it would be a great optional feature!