The problem: some players were deleting foliage and terrain material files to gain a competitive advantage.
The solution: to prevent this the client now sends foliage and terrain material hashes to the server regardless of whether the asset exists on the client. If the client does not have a hash for the file then the server knows it is missing and should kick the player.
The new problem: unfortunately some maps have references to terrain materials and foliage that were removed during development. For example if a bush was manually placed into the map and then the custom bush asset was deleted or went missing due to a workshop file getting uninstalled then the reference to the bush still existed in the map so that it would not be lost if the deletion was accidental.
For terrain materials an easier fix was introduced in the patch: since the server loads terrain materials it can mark a terrain layer as "known missing" so that clients are not kicked when it is missing from both the client and server.
EDIT af