When I started working on cars in the early 1970s when Renaults used to have a sticker on the engine: 'le rincage du moteur est interdit' or in English: 'Don't flush the engine.
I did have a really good American textbook setting out the reasons why flushing wasn't necessarily a good idea, but someone permanently borrowed it.
I've never been sold on the idea of engine flushing, on badly maintained engines the flushed particles could end up clogging the sump strainer with bad result.
Any scheme that requires mandatory engine flushing in order to establish 'Good Garage' status is probably one to avoid.