But under current versions of lemmy, there is no way to block other instances from accessing data from your instance.

Defederating at this point, won’t stop threads copying lemmy.world content and showing it to their users. All it will do is keep lemmy.world users from interacting with threads content and users.

Future versions of lemmy may allow blocking outgoing federation, but until then blocking threads offers little benefit to the users.

Meta can scrape lemmy data. There’s little that can be done to stop that on a public website. Even if lemmy.world was able to prevent sending any data to threads, they could get it another way.