I think car people have declined a bit and the people who are buying the skylines now arent as interested in power and more owning a car they like the look of. Otherwise it will be people who dont DIY, send it to a workshop to be built and then post the result on {Insert cool social media platform}. No need to come on here and ask any questions as its all being done by the shop.
Also i dont think you can underestimate how many people also just go european now for easy, proven ways to modify cars. I recently replaced my skyline with a BMW, and I get the appeal of the european tuning method. i.e. everyone buys the same intercooler, downpipe and intake. the tunes are all made around this and for not much money you get a lot of power quickly, reliably and cheaply. Sure it is not unique in anyway, but its easy. Every knows Stage 1 is X power, Stage 2 is Y and Stage 2+ is Z. If you want to go beyond, everyone buys the same aftermarket turbos and the tune you already own can cater for it. Change the fuel pump? No issue, tune already can cater for the most common aftermarket solution. Everyone buys the same thing so things are cheaper too. NOONE USES A DYNO! its so odd.....
The forums are boring though for the european cars as everyone has the exact same problem. You can almost time it, you know at 160,000kms the charge pipe and a coolant pipe will fail. If someone is having x issue, its always the same fix. Its not exciting, but its reliable (well more reliable than the skyline). The enjoyment is not as high, but its good enough.
So ive gone a bit off topic, but i think thats why you are seeing a reduction in the dyno sheets. people just dont like to DIY anymore.
And agree with mlr, its hard to use the skyline in anger anymore when you know replacement parts are going to send you broke!