In regards to ActivityPub (AP), a big problem is a lack of clarity on what instances are blocked and why by various instances. This is a documentation and accessibility issue as I doubt most instances create or publish either. Much like users picking an instance to join because of a theme or the moderation policies, instances could join a federation because of a grand moderation policy.
The governance of federation is a bit outside of the scope of AP. A fork of AP isn't necessary as a system of trust notaries could overlay any federating protocol.
An ActivityPub instance in a federated network is referred to as a (node) below.
The other big problem is having central authorities like Google or Apple as moderation. You can see a similar problem on the internet with certificate authorities. If a CA is compromised, which they have and they will, then the trust is broken at the root and anything that inherited that trust needs to have it reestablished. I’m not eager to apply blockchain to something that is a governance issue. An alternative is trust notaries.
The general idea is trusting more than one authority, polling them for a consensus on which nodes are filtered/restricted. Many nodes would simply proxy their inherited filters but can change what node they would inherit trust from.
Whitelisting may not be necessary. What's the most a harassing node can do to a large node before it's blocked?
The manual process for communicating blocked nodes to other nodes. A node should be able to subscribe to a larger node for their filters. With multiple subscriptions a node could look for a consensus among trust notaries.
activity pub references
As far as I know,
domain-blocks is as far as mastodon has gotten for inter-nodal moderation.
I remember seeing a list posted by @Gargron of domains that had media restrictions placed on them but I can't find any mention of this in mastodon's documentation.
issues to watch