The policy used to exist, it was Section 3.3.1 of the iOS Developer Agreement in 2010 [1]:
"Applications must be originally written in Objective-C, C, C++, or JavaScript as executed by the iPhone OS WebKit engine, and only code written in C, C++, and Objective-C may compile and directly link against the Documented APIs"
Steve Jobs also wrote some emails on the issue [2], but by September 2010 and following a potential Federal Trade Commission investigation, the clause had been removed again [3].