When I joined back in '70, my unit still had one person who was illiterate & would make his mark + get it witnessed by someone else.
While in the CF, those days are long gone, the equipment is highly technical AND troops are often expected to operate on their own - with only minimal supervision...
... a friend of mine attended the British Army's annual conference of RSMs a couple of years ago. They discussed illiteracy within their ranks - it appears that they had a Sgt who was illiterate but managed to make himself understood. As an example, when preparing a stores request, if he needed a dozen rifles, he'd draw a pictogram of the SA80 and write 12 next to it... not pretty but, it works.
Nevertheless, I'm not sure I could work longterm, with an illiterate Sr NCO.