I think you are missing my point.
The code most recently before => was introduced is IMHO the last version of the SWI-Prolog code that was built using classic Prolog, e.g. syntactic unification (Yes I know SWI-Prolog uses cuts and other things that are not pure but it is mostly classic Prolog that if your read the classic books you should be able to understand). Now with => available, much of the code will change and as time progresses become less familiar to those who can understand Prolog from reading one of the classic books.
When others go to look at the SWI-Prolog source code on GitHub they will now see the use of => and as I.I. Rabi asked
Thus answering questions about => in SWI-Prolog will become a cottage industry. Maybe someone should write a new book for SWI-Prolog so that we can just reference it in the book and not have to answer each question as it arises.
In other words, the version before => started creeping in, which has now changed as you so nicely linked to, was the last classic version and as such should be archived for historical purposes.