FWIW, Facebook does say that they will delete all of your data within 90 days of account deletion. I believe that indicates that they've put the engineering effort to do a full audit of data to be deleted, handle missing references across the product, and to fully delete user data from logs and backups.

From https://www.facebook.com/help/250563911970368 :

> When you delete your account, people won't be able to see it on Facebook. It may take up to 90 days from the beginning of the deletion process to delete all of the things you've posted, like your photos, status updates or other data stored in backup systems.

The case from the article is trickier. My impression is the feature was just implemented with an append-only data model, which is often (maybe usually) a good engineering decision. "Secretly" from the article title feels disingenuous because Facebook never said it was deleted. As an engineer, it's frustrating that I might have to write my software to be more fragile to match the implicit expectations of how a non-technical user thinks software should work. But the frustration on the user's end is also plenty understandable here. Hopefully the gap can be closed a little on both sides by a combination of educating users and being more privacy-conscious in engineering and business decisions.