I think this falls under the part where I said > (assuming the terms themselves are legal in the first place, for example they don't break anti-discrimination

I think this falls under the part where I said > (assuming the terms themselves are legal in the first place, for example they don't break anti-discrimination law) As you mentioned, GDPR explicitly states that you can't extort personal data from people by withholding service. However in the broader sense a company can refuse service if you don't agree to their terms.