I get the feeling that a very useful and potentially life-saving procedure is in danger of falling into obsolescence.
I'm not too worried about that. Speechless code is a very useful tool
in the ATC arsenal of tools and I do hope that ATCOs will continue to be taught how to use it, and get the opportunity to practice it every now and then.
But the speechless code procedure is not something that a pilot can initiate - after all, it's the pilot who loses his voice transmit capability but most of the time he won't even know that until being told so. So initiating it has to be done by an ATCO. Even if the pilot has never heard of the speechless code, explaining it will take all of, what, five seconds? And it's so simple it's practically foolproof after that. At least, from a pilots perspective.
So if all pilots lose all of their knowledge of this thing overnight, I don't think it's a great loss. As long as the ATCOs remember this code and take those five seconds to explain it.
I bet there's more tools in the ATCO arsenal that your average PPL hasn't heard about (or forgotten about) as part of the PPL syllabus, but which are very useful in emergency situations. (As an example, yes, us PPLs have heard of VDF and are supposedly able to request a QDM, but I had never heard of auto-triangulation of every call made on 121.5 by D&D, cross-referenced with primary and SSR radar, until I came to PPRuNe.)