It's a matter of opinion, but as the procedures were only NATO RESTRICTED at the height of the cold war and the procedures can be learnt by listening to RT and watching radar traces, I don't think it matters anymore. Furthermore, as ATP56 now encompasses non NATO countries such as Australia and South Arfrica (NZ sadly departed) the dissemination of the information has now increased to the point it is going to be more widespread anyay.
A terrorist is not going to be able to do much about AR unless they get hold of a MiG29 or an SA2. However, new emerging airforces are developing AR. China has started only within the last month and India will have tankers eventually. These nations will obviously get a few clues from reading an on-line ATP56. But as the RAF has already been conducting AR with the Indians anyway.