Decentralized systems that use append-only logs (which includes but is not limited to blockchain stuff) need the equivalent of a 'git rebase' to purge history.

Decentralized systems that use append-only logs (which includes but is not limited to blockchain stuff) need the equivalent of a 'git rebase' to purge history. Like lwalton says, this is very difficult to do without centralizing authority, as a rebase is a fork, and so you need the whole community to rebase/fork simultaneously. This is a big deal if all data is in the same place. One approach is to break these logs apart, e.g. per-user, like Cabal does it. Instead of replicating one giant log, you replicate many small logs. As a user, you can choose which logs you want to store, replicate, and which contribute to your materialized view. Or, you can delegate this work to a moderator you trust, who in turn may subscribe to other moderators or blacklists to inform their moderation. In this way, there's no centralized authority - everyone is their own authority. Writing the user-facing moderation tools is important work, but is a relatively well-understood problem.