Proton rolled out the beta version of Key Transparency on their own private blockchain, meaning it's not run by a decentralized series of validators, as with Bitcoin or Ethereum. Yen said Proton might move the feature to a public blockchain after the current version serves as a proof of concept.
It's not rewriting history. We're talking about validation of public keys. The exact same information can be added to a public non-beta chain, to satisfy the concerns about security that would come from maintaining a previously private beta chain into production.