Like in a general sense or in the “on going issue” sense?

In general, think of instances like email providers, you sign up on one and you can access all the content from the other providers

But your provider can block or blacklist it’s server from accepting content/users/voting from another provider. This is known as “defederation”. It’s intended to be used against toxic instances like lemmy.ml, Hexbear or Lemmygrad similarly to how email providers will filter another email providers email if it’s gained a rep for spam