GDPR is for companies/corporations to "respect" user's requests about their data.
Lemmy (ActivityPub, actually) isnt a company.
What you are saying is the equivalent of saying that the concept of writing is in direct violation of GDPR.
What you probably can do is request that an instance remove your content... And then do the same for every single other instance of any platform that implements ActivityPub (and not all of them will even have data coming from you) and is federated with your instance. And the only ones that would really need to comply are those that are based or operating in the EU.
This is still the internet, not some magical place.
Use some of the most basic fundamental internet safety rules and don't provide potentially compromising information for no reason whatsoever. Especially since this isnt a corporation such as Facebook or Google who require you do so in order to use their service.