Then these Bitcoin nodes (aka Bitcoin) should not truncate the chain. When you truncate up to some point you trust that you have a valid snapshot up to this time. There is no way to verify the UTXO chain (in the sense that there can be missing transactions) once the prefix is gone. So if someone manages to slip an invalid utxo into the blockchain, and this error is discovered after the prefix is gone, it invalidates every node that forgets prefixes.

So it could work as a fast-bootstrapping probably-reliable mechanism for your home PC. Something between SPV and full wallet. But to be done in full nodes the snapshot mechanism has to be incorporated in the blockchain.

Ok, I'm convinced. As long as the UTXO meta chain is verified by the system, that's fine. I thought initially that it's a separately maintained data structure. If it's integrated in the Blockchain, in the sense that an invalid utxo would cause the entire Blockchain block to be rejected, then it's fine. In this case you're not adding any principles to rely on, and security is essentially intact as far as I see.

I'm not sure who misunderstands. I'll try to rephrase.

Here's the core of my question - does the system (all nodes) forget a prefix of the chain at some point?

If a node reads the entire chain (from genesis), it can prune it locally, sure. But how does a new node bootstrap without the entire chain? It needs to trust a snapshot (rolling root, utxo block, whatever it's called). That's my issue.

I guess I'm not sure what's the ultimate goal of this. Do you want to actually prune the Blockchain prefix at some point, or is this just a mechanism to speed up bootstrapping? My feeling is that this mechanism is secure enough for the latter cause, but not for the former.

To verify that a UTXO set i includes all utxo's, the verifier has to go back to the latest UTXO set it trusts j and make sure there are no missing outputs between j and i. There is no way to do that once the Blockchain get pruned at i.

Technically, it's possible to lose utxo's this way, if the network wrongfully accepts a partial UTXO set and prunes the prefix. That being said, it's quite difficult to take advantage of this vulnerability, so I think it is viable for fast bootstrapping.

Or perhaps I'm missing a part of the mechanism? Please correct me if I'm wrong.