Here are some basic definitions:

Instance: a Lemmy server with its collection of local users and local communities

Federation: allowing users of one instance the ability to participate and interact with the content and users of another instance

Defederation: “blocking” an entire instance and its users from participating and interacting with the content and users of another instance.

Every instance maintains a publicly visible “instances” list where you can see which instances are allowed/federated (listed as “Linked Instances” and which other instances are disallowed/defederated (listed as “Blocked Instances”. That list is always at the same predictable URL for every instance ( https://[instance]/instances ). For Lemmy.World, that list would be at https://lemmy.world/instances.

Instances operators also have the ability to surgically block specific users or specific communities from other instances. This doesn’t mean they have ‘defederated’, it just means they have blocked a specific use or instance. These are considered moderation activities and show up in an instance’s moderation log (also called the “modlog”). Every instance’s modlog is public and visible at the predictable URL of https://[instance]/modlog. For Lemmy.World, the modlog would be at https://lemmy.world/modlog. The modlog has a “filter by action” dropdown making it easy to find certain types of moderation activities. If you search the modlog for “removing communities” you can see the communities that an instance has removed or blocked.

In the case of the piracy communities, they were removed from Lemmy.world, but federation still exists between Lemmy.world and the other instances where those blocked communities still exist.

You specifically cite the example of piracy going away as a reason for wanting to compare instance’s defederations, but that activity had nothing to do with defederation. Lemmy.world is still federated with the instances that hosted the piracy communities.