It is true that the many FAA and independant U.S. publications state that an instruction to fly the runway heading means to steer the actual published magnetic measurement and the pilot should not to allow for drift. I've always considered this as odd. Aircraft climb at different speeds and therefore will experience different rates of drift. First, the climb path is designed to give maximum separation from obstacles and not only from other aircraft taking off on a parallel runway.. CFIT forms a large part in the accident statistics. An aircraft climbing at 80 kts with an average cross wind of 20 kts will drift 15 degrees. Another aircraft climbing at 120 kts will drift 10 degrees, so a convergence factor of 5 degrees. At a stronger crosswind of 30 kts the convergence increases to an alarming 22 degrees and 15 degrees respectively, so a convergence of 7 degrees.
Within the UK such instructions are not used by ATC.
CAP 413 edition 23 published 9th April 2020
4.37
An Omnidirectional Departure procedure is designed on the basis that an aircraft maintains runway direction until it reaches such a height that it can make a turn in any direction and maintain the prescribed obstacle clearance.
“BIGJET 347 Omnidirectional Departure runway 27 on track REVTU climb Flight Level 80”
4.38
Local departure instructions may be given prior to the take-off clearance. Such instructions are normally given to ensure separation between aircraft operating in the vicinity of the aerodrome.
“BIGJET 347, after departure track extended centre line.”
“BIGJET 347, after departure climb straight ahead.”