Plus it is part of discussion and that is the ground functionality of a forum. But if there is purpose to send ads or anytyhing else that is outside the main scope of a forum then consent is needed for that.
- My e-store can and will send emails connected with transactions and useraccount — technical and legal stuff — and a user hasn’t other option than accept. I don’t need even ask, because the law says I must send those. But I can’t send marketing emails without given consent.
- Wordpress has own mailing list and because it isn’t automatic, but needs joining, I can send (almost) what ever I want.
- Discourse takes care of commenting of the Wordpress, plus other discussion. As said, I can send automated mails of topics, but nothing else (sure, there is obvious loophole…)
That is not actually difficult at all.