The dirtiness information, used to perform clever and efficient updates of the pathfinder, was handled in a very hacky way. Three components work together: the obstruction manager keeps track of small changes that don't warrant a full update, the pathfinder keeps track of terrain updates and performs the clever updates, and the AI manager must perform the same updates on its own pathfinder asynchronously with the pathfinder (the AI manager passes data to its worker once per turn).
The previous system was obfuscated, badly commented and unpractical to maintain. Additionally it included a bug that was triggered during water rising in Extinct Volcano, resulting in an OOS on rejoin (the rejoining player would have the correct passability map whereas the host would suffer from incomplete pathfinder updates).
The new code is cleaner, much easier to understand and to fix in case of bugs (notably this one OOS, which is fixed here). The obstruction manager <-> pathfinder and the pathfinder <-> AI interaction are better compartmentalized and should work regardless of what the modders do with their terrain updates...