You mean as instance blocking? Because the Lemmy devs have stated that it's not going to work the way everybody's assuming it's going to work.

So far the way that it's been laid out it'll only block communities on that Lemmy Instance, users will not be filtered.

That's ignoring the fact that Lemmy's blocking system is already flawed in it's design and isn't really an effective tool against malicious users.

So we really shouldn't treat blocking even of instances as personal defederation, because it isn't and unless something really changes and Lemmy's development it never will be. You can on Mastodon because Mastodon's blocking system is much harsher as well as the fact that federation highly depends on following, but lemmy works much differently and also has a significantly weaker blocking system (I should also add it does not respect mastodon's blocking system) so because of that being able to block instances should not and cannot be considered an alternative to defederation, especially when it comes to malicious instances.