Data in Bitcoin is undeletable, it's impossible for any law to force anything from being deleted on Bitcoin. Then the same exceptions that apply there would apply to Lemmy since the technology is similar in the relevant aspects (besides deletion being theoretically possible on Lemmy).

As for Meta, the problem is that the data they're sharing is not public. Meta is not getting fined for sharing things you posted on your publicly, since they share those regardless by virtue of them existing and being publicly available, they're fined for sharing things you put privately or data derived from non publicly available sources such as how you interact with Meta.

Any information that a user willingly makes public can be processed in any way, even if it includes identifiable medical information (which is the biggest no-no of GDPR). It even has a specific point about it in 9.2.e

processing relates to personal data which are manifestly made public by the data subject;

Essentially saying you can process anything that was made public by the person. GDPR is to protect people from companies doing shady things, not to prevent people from themselves. Because EVERYTHING is public in Lemmy, all data in it has been manifestly made public by the person who created it.