I was encouraged by a "Contractors Agency" to join one of these schemes some years ago.
The idea was that the Contractor and the Agency would "split" the tax "savings".
I smelt a rat and declined their "thoughtful" proposal - I am damned glad that I did
I don't think in practice that anything was "retrospectively applied"; I believe that what has happened is that HMRC has reviewed some of the creative schemes that were dreamt up by tax advisers and decided that some of them were never actually legal - I think that there was a tax avoidance scheme used by many people in the entertainment business that involved financing films.