Is providing forex deal recommendation services and creating and selling computer programs that aid in forex trading considered unlawful if the majority of traders use the forbidden margin system, based on the principle of aiding in sin?

As long as the questioner predominantly believes that most users of the program will use it for impermissible margin trading, it is not permissible for him to do so. This is because the predominant belief is that most traders will deal with it in an unlawful manner, and anything that is used unlawfully, or is predominantly believed to be so, is forbidden to manufacture, import, sell, and promote.