It's a matter of public policy: one cannot expect property-owners (in this case) to be afraid to fill gopher-holes after an accident until the statute of limitations has expired, or the court case is concluded.
But it must be admissible as evidence if, under the circumstances you describe, I was defending the action by claiming that there was no gopher hole, therefore no possibility of the accident having occurred as alleged. If they say "Oh yes, we changed the routing out of an abundance of caution, once we learned about it," then your principle of law would apply. But if they altered it in an attempt to deny that they directed them to use that route then that is "guilty knowledge."
In this case it really wouldn't be the offense, but the cover up, that controls admissibility.