> I have seen there is a "Installation quick guide" (https://docs.moodle.org/36/en/Installation_quick_guide) that seems to be installing Moodle 3.5 LTS which is what I want.
You mean https://docs.moodle.org/35/en/Installation_quick_guide, not /36/, right?
> ... when I installed Moodle a few years ago I instead followed the "Step-by-step Installation Guide for Ubuntu" (https://docs.moodle.org/en/Step-by-step_Installation_Guide_for_Ubuntu).
That one too gets redirected to https://docs.moodle.org/36/en/Step-by-step_Installation_Guide_for_Ubuntu and it was not there a few years ago!
> at the top of it it says "Note: This document is about installing Moodle 3.1 ..." and the git checkout command line in the document uses MOODLE_33_STABLE
The Moodle Docs has some magic build in to it, which fails to catch such subtleties. The forum Moodle community sites https://moodle.org/mod/forum/view.php?id=7135 is the official place to ask. Be warned, the common answer is, Moodle Docs is a wiki, repair it yourself!