Ok, so for the semantic morons out there, perhaps 'legislated' was not the best word choice. In my defence however, I refer you to Gen 0.1-4....
4.6 Throughout the AIP the term “should” implies that all users are encouraged to conform with the applicable procedure. The verbs “must” and “shall” are synonymous and mean that the applicable procedure is mandatory and supported by regulations or orders. The word “must” is preferred over “shall” and is used almost exclusively throughout the AIP Book.
For example:
IFR RPT and CHTR aircraft which are required to be crewed by two or more pilots must be fitted with an approved airborne weather radar system.
The rules are clear.