Agree! I think they started using plastic on everything when the SBS models came on the scene.
I believe the first plastic stock and trigger guard was on the M3 'Professional' which pre-dates the SBS - I got mine in 1995 but they were around for a few years before that. Accurate rifle with a great barrel and action, but its not just the trigger guard, its all the bottom metal that was replaced by plastic. It is a one piece out unit that extends from the front action screw to the rear screw behind the trigger guard and includes the magazine surround that the plastic rotary magazine slots into.
I am on my 3rd unit - replaced one when I discovered that plastic and bore cleaner doesn't mix well, and another because it cracked after a particularly excessive load testing session shooting from a bench. It is a great shooting rifle, but would be even better with a laminate stock and metal magazine and bottom metal!