Looking for writings about lifted and unlifted types, I stumbled upon this page called Informal Semantics. It looks like it has belonged to the GHC user guide some while ago. But I could not find it in the later versions of the guide.
Looking at the history of the GHC repository, I see that the commit 5bf22f06ef71f61094de7564dee770f136d5481a Remove external core(dated back at 2014) removed this page.
My guess is that it was just becoming increasingly outdated. Even that page you linked lists it as explicitly for GHC 6.10 while it is part of the GHC 7.8.3 user guide.