Imagine some engineer writing some feature you've never heard of and they decide they need data in a different format. They write a job to do this.
Now imagine that job somehow has a bug which prevents the deletion system from understanding that copy exists. That bug could be non-obvious to find.
It does if you have cold backups that take a year to cycle out. They're often offsite, compressed, and incrementally hashed so finding individual items and removing them is really hard -- you're much better off just waiting for them to expire, and a year isn't an unreasonable amount of time.
Backups are covered under GDPR, although when a user requests erasure you can say "your data will be rotated out in X months/years". Not sure how this applies to access, but I assume it's similar: https://ico.org.uk/for-organisations/guide-to-data-protectio...
'Your delete request is being processed. No one will be able to see this item though it may take up to three days for the data to be completely removed from our servers'
This is not a technical challenge, it's an issue of priorities and Facebook doesn't prioritize privacy. They were able to release a TikTok clone in a few months, but can't solve deleting data?