"A wild exaggeration. There are more & more places being closed off that were formally available. My guess is that they have been abused by campers."
This is spot on. Up through probably the mid-90's, a road "Camping" trip on the loop (Anchorage-Fbks-Valdez-Anchorage) didn't need any planning since one could pick any wide spot in the road and "camp" there overnight. Similar option for the southern leg (Anch - Seward-Slodotna-Kenai-Homer, etc). In the 90's, lots of road-building / road improvement work took off and many of the wide areas became access to staging areas for construction, or became official pull-offs or became waysides or whatever and were either physically blocked (gates or jersey barriers) or had signs emplaced "Construction traffic only". That continues to this day along with the increasing number of signs popping up in the otherwise unaffected wide spots "No Camping".
Another not insignificant impact to the "stop anywhere" paradigm is that more and more of the roadside property (this seems a bit more prevalent on the Glenn and Parks than on the Seward Highway) is being either bought or actively occupied and what was previously suitable to pull off now bears either housing or "Private Drive" or "Private Property" signage.
One trigger event was the (unfortunately fairly frequent) dysfunction of the Alaska legislature in the early 2000's where funding was not given to the agencies (DoT/PF, State Parks, other DNR) to support the then-designated "camping spots" (often just a flat spot with a well and outhouses) so those spots were blocked or locked. People then just went to the next wide spot and "camped", leaving trash, garbage, s**t, and all sorts of debris. The next legislative cycle saw a fair amount of $$ dedicated to the "no camping signage".
The wide spot in the road camping paradigm was indeed abused by campers.