The GDPR specifies that opt-outs are also retroactive, but of course we know that all corporations happily follow the law.
Disregarding that, laws don't apply to the first processing party only. If you keep data, that you got informed are not consented to anymore, it is the same as if you keep selling fenced goods.
Of all the reports I've submitted (evidence included) followups and fines have been issued to exactly 0 companies. Hell, Quadlock [phone mount company] happily acknowledged that their policy is to verify identity by requesting plain emails including photocopies of the credit card used for purchase and full state ID. Absolutely against regs.. who cares? Not the SCC nor the FTC.
Noyb alone has several hundred successful GDPR lawsuits: https://noyb.eu/en/project/cases