That would be illegal as per the GDPR - you can’t force consent by withholding service. They have no choice but to stop ad targeting based on personal data and

That would be illegal as per the GDPR - you can’t force consent by withholding service. They have no choice but to stop ad targeting based on personal data and instead only use publicly available data (public actions the user takes on the platform such as liking a page, commenting, etc) unless the user explicitly consents to their personal data being used for ad targeting.