You were on a plan that explicitly includes ads; if you don't want to see them (I don't either), then you can either upgrade to the no-ad tier or, as you did, cancel. Neither choice is wrong, but OpenAI definitely has been open about that lowest-tier paid plan having ads.
Never pay to avoid ads. Signals you've got money to waste, makes your attention even more valuable. You're paying to self-segment into the upper echelons of the market.
Or you go golden platinum extra special customer - to become even more addless for now.
I find it hilarious that they insert the adds that manual though. It would be so easy, to generate subconcious addds with an LLM. Like weave a slogan reminder into the conversation...