I've seen written elsewhere that Burt did that with design #54 (Quickie).
My guess is that despite his vast achievements, the RAF homebuilt designs still hold a place dear to his heart.
Consider this though: as long as Burt continues to passively hold the rights to the designs, he affords anyone the freedom to build those designs and fly them, providing they take full responsibility for the consequences. That's altruism at its finest, and neatly gets around the problems of living in a litigious society.
I think we owe it to him to continue to build his designs, out of respect for a pioneer who has shown remarkable wisdom in his actions.