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.

[1]: https://mpv.io/manual/

[2]: https://wiki.archlinux.org/

[3]: https://developer.mozilla.org/en-US/

[4]: https://www.postgresql.org/docs/