Maybe I'm naive but I don't like broad defederation from a platform. That's what will ruin the dedicerse when the money and large companies creep in. Imagine Sony vs Microsoft video game wars with exclusives. It's a system designed for decentralized enjoyment of all. Banning/blocking instances from other instances by default is problematic to the health of the basic idea of federation.

Leave the decision in the user's hands, IMO.