Reposting what I posted here a while ago.
Companies abiding by the GDPR are not required to delete your account or content at all, only Personally Identifiable Information (PII). Lemmy instances are unlikely to ask for info such as real name, phone number, postal address, etc; the only PII I can think of is the email that some (not all) instances request. Since it’s not a required field on all instances, I’m going to guess that the value of this field does not travel to other instances.
Therefore, if you invoked the GDPR to request your PII to be deleted, all that would need to happen is for the admin of your instance to overwrite the email field of your account with something random, and it would all be in compliance. Or they could also choose the delete your account, if they prefer.
Source: I’m a software engineer who was tasked at some point with aligning multi-billion-dollar businesses to the GDPR, who had hundreds of millions of dollars in liability if they did it wrong and therefore took it very seriously. I am not a lawyer or a compliance officer, but we took our directions from them directly and across several companies, that’s what they all told us.