Delete means mark as available. Not gone. Available. The file system stops pointing to that sector of disk and allows something else to overwrite it eventually, on its own schedule, when the space is needed. Until then the data sits there exactly as you left it, unlabeled, accessible to anything with low-level read access and a reason to look.
This is not a flaw in storage systems. It is a design decision, because actual deletion is expensive. Overwriting every sector takes time. TRIM on SSDs is asynchronous. Backup jobs run on their own cadence and do not check whether you wanted that file kept. The cloud synced before you hit delete. The email left the server the moment you sent it, copied into the recipient storage, their provider archive, and any compliance system that happened to be watching the pipe.
Opting out carries the same accounting. You can delete your account. The platform will confirm it with a sad graphic. What you cannot delete is the derived data: the behavioral profile built from your activity, the graph of who you knew and how often, the model trained on your content before you left. You opted out of the interface. You did not opt out of what was learned from you.
None of this means deletion is pointless. Reduce the surface. Use full-disk encryption so the unlinked sectors are ciphertext. Request deletion under whatever regulatory framework applies to you and document that you did. Make the cost of recovery higher than the value of the data. That is the practical ceiling. Understand it clearly and work within it, because mistaking the ceiling for the floor is how people end up surprised.