I don't know if the whole Order of the Phoenix would have been aware of the plan. For a start, Hagrid and McGonogal are shocked at the turn of events with Snape killing Dumbledore. Unless they were merely acting at it... but I don't think so. However, it's possible that it was a plan between just Snape and Dumbledore and that it was to be the top secret plan. Thus, requiring Snape to remove it when he was teaching Harry occlumency.