Yes, the applications you build using the FOEX Plugin Framework will continue to run even if you decide to cancel your license subscription. By discontinuing your FOEX license agreement, you will no longer receive updates, hot xes or any other future improvements that we'll bring to the FOEX Plugin Framework.
Furthermore, we do not guarantee, nor will we provide support for any compatibility issues that you may encounter if you decide to use another version of Oracle APEX, other than the one initially used for developing the application(s).