No, you misunderstand me. I'm proposing to delete all the obsolete material and replace it with your recommendation of the BLFS book.
It is OK to delete the obsolete material. I think that it is all covered in the BLFS book for version 4.10.x. There are some more obscure dependencies that are not covered which I could add some notes on those that I have knowledge on from personal experience.
Putting the recommendation on the BLFS book there to replace it would be redundant because people installing 'Linux from Scratch' would have already been to the website and already be familiar with the book.
There are also people that are not using the LFS distro that just want to build KDE from source (for various reasons). What I want to do is to suggest to them is that a source of instructions on how to do this is to be found in the BLFS book. That is, unless there is an equally detailed tutorial on how to build (the release version of) KDE from source on KDE Techbase. My tutorial, which appears to have been deleted, was not nearly as detailed or as complete (but at the time it was written, the subject was not covered by BLFS).