This forum potentially sends out summaries and stuff like that, thats why it is asking for consent.
Eventually, you can configure the level of email notification to your liking (and if I saw it correctly by default it should not send anything).
I cannot deactivate, neither do I even care about it. This is default software, we're not gonna reprogramm it because someone is afraid to consent to something that is a legally delicate topic. If you don't trust the FAForever administration to not spam you via a fucking forum, stay out of the forum or stay out of FAF, I don't care.