In the stock Lemmy web client there is apparently no mechanism for users to fetch their history of posts. The settings page gives only a way to download settings. This contrasts with Mastodon where users can grab an archive of everything they have posted which is still stored on the server.

Or am I missing something?

IIUC, there is no GDPR issue here because no data is personal (because all Lemmy accounts are anonymous). But if a Lemmy server were to hypothetically require users to identify themselves with first+last name, then the admin would have a substantial manual burden to comply with GDPR Art.20 requests. Correct?