Thought about it a bit more (I'm bored at work) and decided to add some to my previous message.
Judging by Nenkai's messages, the new track been at work and was kinda finished last month (if I remember correctly), and I assume it means there are no other "ready to go" tracks in the game files right now. That can technically mean they are actually just very slow with making new content now and not just holding back a bunch of stuff. Harder to determine that with cars though, since I don't think we ever heard about cars being ready but not released, but we have a long list of cars which have been actually appearing in the game, and Nenkai also mentioned cars "being worked on", so probably it's the same situation as with tracks, they do work on a bunch, but none are really ready to be released and just sitting in a waiting room for dripfeed purposes. However, when Silvia S14 was added, he said there were no mentions of it before the update when it was added, so can't really be any sure about this, since we don't know when that Silvia was actually finished.
What I'm saying is, it kinda seems like PD possibly actually just reduced the amount of content they produce, rather than just holding the content back. Back in GTS days after PD started using outsource it was possible to find here and there different posts from 3D modellers saying something like "I've worked on this car for Gran Turismo and it was just released today in a new update!". There are really not enough evidences to be certain, but it is possible PD actually just dropped outsourcing again and that's why GT7 gets bit more content than GT5/6, but nowhere near GTS. Or they are just already working on a PS5-only GT8 and want to introduce as much new content for it as possible, lol.