I was only talking about this a few days ago. I've ridden pillion for the last 25 years and have really noticed the decrease in general nodding these days. Now I'm on my own bike with L plates I've noticed how many people don't nod anymore. Some still do but the majority don't.
Recently, I mentioned this to a friend of mine and went pillion on his Duke streetfighter where I proved my point. He made a conscious effort to exaggerate his nod as we passed bikes and only half of them responded.
I think there are more scooters on the road now than there have ever been and I'm sure that might have something to do with the decrease in nodding.
I find ped riders don't nod, seasoned bikers are hit-and-miss.
Also, the old nod, you know, straight up and down, has changed into a winky side-head-tilt but without the wink ;D who the hell started that? ;D
I don't care, I nod anyway, btw, but it does feel, to me, like L plates aren't worthy of a nod