From June 30, 2022, Apple mandated that any iPhone or iPad app, which allowed a user to create an account inside it, should also allow the user to quickly delete their account and ALL associated data. This applied even if the app user did not use the app to create their account but created their account on the web.
So if you have difficulty deleting an online account just download the iOS app of that website or web account. Then open the app and go to settings and use the Delete Account button.
According to Apple, deleting an account removes that account from the developers records with any data associated with the account that the developers are not legally required to maintain.