Following the announcement by beehaw admins to defederate from lemmy.world and sh.itjust.works, there has been many posts and messages regarding that decisions and what other instances will do.
I personally believe Lemmy/kbin can only thrive if there is a free flow of content between different instances, with instance admins taking a back seat and focusing more on the infrastructure and making sure the technical bugs are smoothened out. Community mods can moderate their communities, and users can block the communities they don't find appealing (there's even a toggle in settings to hide every NSFW post from your feed altogether).
We don't want to create walled gardens, nor do we want to make Lemmy more confusing than it already is for new users. We will not be defederating from any instance if there is even one good community on it that our instance users might find useful. So far we have only blocked lemmygrad.ml, and right now we have no plans to block anyone else.
I sort of agree, but I also know that as Lemmy becomes more popular, defederating will become necessary as trolls and hate groups open instances. It's a problem on Mastodon. A trans friend there posted about her experience bra shopping and a bunch of transphobes from other Mastodon servers came in to attack her, including an admin of one server who called her a slur and told her to stop reporting because they'd never remove anything hateful towards trans people.
On a centralized service like Reddit, hate subs can just be quarantined by the admins or removed wholesale. With a decentralized service, every instance will need to defederate those groups to keep them out. There's no way to bar them from making a new server.
I think it's different with Lemmy since people make posts in communities and it's in the community moderator's power to remove problematic comments.
If an entire instance supports or encourages such behaviour though, then you're right and such instances should be defederated.