Citation? I think this is not true. The EU law seems to just require explicit consent. So websites and apps can ask whether user wants to share data to keep using them for free or pay a subscription fee.
A "consent or pay" model is sadly widely used, but it's at least very controversial and probably illegal. No data protection agency has gone on record to say it's definitely illegal and no fines have been given out IIRC, but the EDPB had some tactfully negative things to say about it [0], and the Czech DPA has ordered at least one company to cease the practice in a preliminary ruling [1]. (Which the company seems to be completely ignoring, as is sadly common.)
Article 7 https://gdpr-info.eu/art-7-gdpr/
Recital 43 https://gdpr-info.eu/recitals/no-43/
Those two in combination stop companies processing data for unrelated task to the services they provide. And it's indeed true and already been applied, see this: https://www.digitalguardian.com/blog/google-fined-57m-data-p...