EGPFlyer provides a verse of regulation that applies in the UK and most likely to UK registered aeroplanes globally, within the scope of the question.
Smythe, by all accounts, quotes a rule for the design of departure procedures. While it exists for a good reason, it is outside the scope of the OP's query.
Useful debate on both sides but perhaps also worthwhile keeping the two baskets separate.
Directly from the AIM link provided:
All DPs provide the pilot with a way to depart the airport and transition to the en route structure safely.
Pilots operating under 14 CFR Part 91 are strongly encouraged to file and fly a DP at night, during marginal Visual Meteorological Conditions (VMC) and Instrument Meteorological Conditions (IMC), when one is available. The following paragraphs will provide an overview of the DP program, why DPs are developed, what criteria are used, where to find them, how they are to be flown,...
.
My point being, the rules how to fly a DP only apply if the pilot needs or chooses to fly one.