The "infamous" reference generators from NIST 800-22 included linear, quadratic and cubic congruential generators only. A potentially vulnerable implementation that may have used this document as a reference would probably have only gone up to the cubic case. So I think it's unlikely that someone used a recurrence equation of higher degrees. But you never know. Also, the higher the degree, the more resources the attack will require. So, we opted for a balanced cost/benefit approach.