> My source for this comment is the Linux Device Drivers Development book from about 2004 still being the primary
source of information on linux development
The kernel has never had any official development guide or book. Various tutorials and howtos and books come and go along the way. Another good one,
https://www.kernel.org/doc/gorman/pdf/understand.pdf
They don't really scale at all. The device driver book scratched the surface of device drivers even back then. You can still read things like that and get an idea of the concepts which remain pretty similar, but the best way to really learn it beyond hello world stuff is getting into the code. If you want to write a device driver, start with a copy of a different driver of the same class which is well maintained, for example.
> and when asked about an updated version the response was something like "Back when it was written we needed more developers on linux but not anymore".
Not sure of the "we", but Linux kernel developers are in pretty decent demand today and the number of paid contributors seems to be pretty well growing. Certainly a lot since 2004.
> As well as the comment from linus which I can't find anymore where he says other OSS projects need help more than linux.
Yes that has long been true.
I'm not saying you're simply wrong, and I'm sure some people would find great benefit in more or more up to date books on it, but there has been no point where developers just decided that's enough of making Linux more friendly to newbies or have an easier learning curve. Nor has there ever been a great deal of effort to make it easier for newbies mind you. But as I said, people are always trying to clean things up and improve docs and add more seat belts trying to do staging and mentoring and things etc.... but it's just a difficult thing to improve.
My source is only my anecdote of contributing to Linux for ~20 years, so I admit I could well be biased or blind to some of these issues.