Piracy

One of the best strategies on the internet is to assume that everything will leak eventually and operate with that perspective in mind.

If you are actually concerned, browse with a vpn, sign up without an email.

The amount of work needed to get any extra info is not worth it and hard to actually do anything with.

It really depends on how that specific instance is set up. Of course during the registration you provide your username and email, but some instances may keep logs that contain things like ip addresses, your user agent and so on (my instance does, for example, but only for a limited time), while some may opt to not keep that info at all (from the sysadmin perspective though they'd kinda be in the dark while trying to fight things such as server abuse).

However, I don't know how it looks from the legal perspective. I'd imagine only the courts could force you to share the information stored on the server, but that requires a process.

Well, there are not many details to share anyway (registration is just username/password, e-mail is optional). And since everything is federated, they'd need to bring down all of Lemmy, which may not even be possible for them due to different instances being hosted in different countries.