Why are almost all older versions yanked?

The vast majority of the older versions have been yanked: https://crates.io/crates/rmp-serde/versions. Surely they don't all have critical vulnerabilities that warrant this?

I can obviously use a lockfile to retrieve the yanked version, but this broke underneath my feet. I have a few libraries which don't have lockfiles per the previous rust guidelines and are depending on 0.x.y and can't be upgraded to 1.x. So these libraries just stopped working out of the blue and now require a lockfile going forward, which is concerning