In the November 2016, in
our VK group for the first time appeared an
entry with an archive of Half-Life 2's original map sources (you can download them here,
on our site). This map sources are interesting because unlike decompiled maps, they contain all the unused VIS groups that contain lot of cut, unused or changed content.
Today, Livelong2, and admin of
VCC Wiki finished an
article about ALL of such groups, with handy navigation based on the locations and screenshots of each cut visgroup.
We recommend this article both for mappers who look into this map sources and those who just interest themselves in a content cut from levels.