A friendly reminder: most of us here on Meta are not lawyers. For the few users that are lawyers, they’re not your lawyer. Arnoud (from the article Richard linked) isn’t your lawyer either.
We’re happy to discuss the technical side of handling GDPR data erasure requests. We can also discuss the community management side of it. We cannot provide answers to the legal side. For that, a lawyer, specifically your lawyer, is needed. One that knows both GDPR and the specifics of your circumstances. Any information provided here is not, and must not be considered legal advice.