Of course it's a grey area. The position Lancashire police take (as I've shared before) is this, and I paraphrase: "Just for the sake of clarity 'holdbacks' haven’t been used for a number of years now, unless they’re spontaneous, can’t be pre-planned owing to ECHR Article 5 - Right To Liberty & Security. Pre-planned holdbacks could be seen as 'unlawful' detention so are avoided (generally)."
As I understand it, it's not the Club's decision to make (which is why it isn't in T&Cs). I believe it is a Police decision on a game-by-game basis, there to be invoked spontaneously by Match Command if it is felt the circumstances on the day merit it. I don't know if the Club will challenge that but right now we have to work on the assumption that it is not a regular or permanent solution to managing the exit of fans from BR and through the surrounding streets.