I wouldn't make a blanket statement either way, but there are certainly counterexamples to this:
- The mpv manual[1] is a work of art.
- The Arch Linux wiki[2] is a treasure trove of information for not just Arch-specific topics, but Linux in general.
- MDN[3] is the defacto standard for any web documentation.
- The PostgreSQL[4] documentation is quite thorough and high quality.
What I think backs up your point are two things:
1. There's just a vast amount of software since those early days. "Software is eating the world", and it's realistically impossible for most of it to be well documented.
2. A lot of information is spread out in books from users of the software, on blogs, tutorials, forums, videos, etc. Sure, this might be seen as a failure of the software authors to produce good documentation, but many of these resources didn't exist before the web made them accessible. In some ways this is better than having a single source of reference, as you can benefit from the collective wisdom of the hivemind, rather than only from what the author thought relevant to document.
[2]: https://wiki.archlinux.org/
The thing is... languages were smaller because they hadn't started the accretion of thousands of libraries and frameworks.
In my experience, every language out there is somewhat easy to learn and master. The ecosystem around it is an insane and ever-growing Katamri Damacy of (largely) crap. We all must know it, must use it, and must contribute to it because no one trusts the work of the individual and only the work of an aggregate of individuals... people often don't even trust their own code.