Without some sort of reproducible builds (which are really finnickey to actually get) this doesn’t really help though. Adding some set of malicious patches before doing the binary release is trivial.

600k seems a bit too much. That’s a fairly large city.

Blockchains have the property of being append-only, so a blockchain is precisely what makes it impossible to delete transactions. That being said, in a distributed system, once the message leaves trusted servers, it is obviously also impossible to delete it.