When did the NEC code start to require the separation of neutral and ground bus bars after main disconnect?
You're after NEC 250.32(B)(1)

The applicable NEC section is 250.32(B)(1), which reads as follows ("equipment grounding conductor" = "ground" and "grounded conductor" = "neutral" for us, and exception 2 is irrelevant for this):


  (1) Supplied by a Feeder or Branch Circuit. An equipment
  grounding conductor, as described in 250.118, shall be run
  with the supply conductors and be connected to the building
  or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used
  for grounding or bonding of equipment, structures, or frames
  required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any
  installed grounded conductor shall not be connected to the
  equipment grounding conductor or to the grounding electrode(s).
  
  Exception No. 1: For installations made in compliance with previous
  editions of this Code that permitted such connection, the grounded
  conductor run with the supply to the building or structure shall be
  permitted to serve as the ground-fault return path if all of the following
  requirements continue to be met:
  
  (1) An equipment grounding conductor is not run with the supply to
  the building or structure.
  
  (2) There are no continuous metallic paths bonded to the grounding
  system in each building or structure involved.
  
  (3) Ground-fault protection of equipment has not been installed on
  the supply side of the feeder(s).
  
  If the grounded conductor is used for grounding in accordance with
  the provision of this exception, the size of the grounded conductor shall
  not be smaller than the larger of either of the following:
  
  (1) That required by 220.61
  
  (2) That required by 250.122


The current state of this section was reached in the 2008 NEC, which forbade new construction from using the feeder neutral to provide ground for the fed system.  (Your choices now are to provide a separate ground wire, which is what is normally done, or use a distribution transformer to create a separately derived system for the fed building, which has its own N-G bond.)  Many of the installations that fall under the current exception were installed prior to the 1999 NEC, which is the first edition that forbade parallel current paths between the structures involved.