This is about account deletion, not cancellation. But cancellation is also a fun topic in its own right. I don’t know about Germany, but cancellations are a solved problem here in Austria, even accounting for shady business practices. 3rd-party services exist that fully automate the cancellation process for most cases. They email the company, send another reminder email, store the email server response as evidence for court, and submit a complaint to the responsible Schlichtungsstelle, which then light a fire under their ass to cancel your service. If they’re retarded enough to not cancel your service, then you can always take them to court with the stored evidence.