That is untrue. @Jagster is correct, it’s not legal in EU.
The Dutch telecommunications law requires that the user gives explicit and separate permission for newsletters, and the GDPR has a different ground for processing for the forum and the newsletter. Where the forum can be seen as performance of a contract, the newsletter requires explicit consent.
So you’ll need a separate checkbox at least. And if I remember correctly there is jurisprudention in NL that the checkbox need to be unchecked by default.
That all said, what a great plugin! 