During 10 years in professional flying I have met very few who are even aware of the speechless code and this is especially prevalent amongst the 'new generation' of (UK) co-pilots. Most of the cognoscenti that I have come across are ex military, a world in which a 'practise speechless' was de rigeur to keep ATC and the runway caravan controller current.
I get the feeling that a very useful and potentially life-saving procedure is in danger of falling into obsolescence.