They are thoroughly indoctrinated and the only way they can go against their natural instincts is to not see you. Your father has gone the cowardly route by sending you a letter because he doesn't have the heart to do it in person. I'm sure he and your Mother were cringing as they dropped it into the mail. I suspect that if you respond, anything you say will fall on deaf ears and may make him feel justified in his actions. I'm guessing that if you remain silent, they will not only be eaten up with curiosity and guilt but their natural affection will eventually override whatever has triggered this sudden hard line they've taken.
As hard as it is to do, you can give them what they've asked for and if you do, I wouldn't speak of this with anyone you know who may report to your parents what you've said or what your reaction to all this is. Let it be as if you've disappeared off the face of the earth...total silence.
You could pretend that you didn't receive their letter and call them or visit them as if nothing has happened. Let them tell you to your face that they want to cut ties with you. Don't defend yourself or discuss the matter. Simply have a brief calm response prepared and make your exit.
You: Just so I understand....you don't want to hear from me unless there's a family emergency such as when one of you eventually becomes seriously ill or is hospitalized.
Them: Well yes...
You: It hurts me that it's come to this but I have no choice but to honor your wishes. (say goodbye give them a hug and leave immediately)