This is not the engine code, this is the game code that was released even prior to Quake 2 being GPLd.

    -----------------------------------------
    John Carmack's .plan for Dec 11, 1997
    -----------------------------------------

    The Quake 2 public code release is up at:

    ftp://ftp.idsoftware.com/idstuff/quake2/source/q2source_12_11.zip

    This source code distribution is only for hard-core people that are
    going to spend a lot of time pouring over it. This is NOT a
    how-to-make-levels-for-q2 type dsitribution!

    This should keep a bunch of you busy for a while. :)

https://github.com/ESWAT/john-carmack-plan-archive/blob/mast...