> Given the ability to manipulate gitnhistory I am not sure if a simple time based revert is enough.

Rewritten history is not a real concern because it would have been immediately noticed by anyone updating an existing checkout.

> Overall the only safe action would IMHO to establish a new upstream from an assumed good state, then fully audit it. At that point we should probably just abandon it and use zstd instead.

This is absurd and also impossible without breaking backwards compatibility all over the place.