Can a server script change the local player's properties when FilteringEnabled is turned on? I know a local script can't change the properties of objects in the workspace when FilteringEnabled is turned on, but does it apply the other way around? In other words, the local player cannot make changes to the server while the server can make changes to the local player?
A server script can access and change players properties; it cannot, however, from a local script. In server scripts you can't access LocalPlayer, it's better to do an OnPlayerAdded function to get players. If you need send data/info from client to server, use remote events/functions.