No-one actually knows. What constitutes erasure and how to deal with all the edge cases around backups, archives, unstructured data and so on is one of the big ambiguities under GDPR, and one of the areas most in need of (but mostly lacking) actionable guidance from official sources.
The law might have been around for 2 years, but in practice many smaller organisations didn't find out about it until a few months ago. Much of the official guidance is more recent even than that. And much of that guidance doesn't really help anyway, because it is often almost as vague and/or incomplete as the original regulations themselves. Questions about numerous everyday issues that will affect literally millions of businesses across the EU are still lacking useful answers.
