Actually, PJ, I'm not sure if you can really say that. I mean, I've collected a fair number of 3e adventures, from Dungeon, and various other places and there's a lot of adventures out there with less restrictive maps.
Heck, the World's Largest Dungeon is about as unrestrictive as you can get.
I'm not really sure if you can point to a period where maps get more or less restrictive. Heck, DL 1, the first Dragonlance module, much maligned for its railroading plot, has one of the least restrictive (and very pretty) dungeon crawl maps you can find.
I wonder if its simply a case of numbers. Up to 1982, 1983, you don't have all that many modules. Compared to the absolute explosion of modules afterwards. I mean, heck, just in Dungeon, you have hundreds and hundreds of 3e modules, never minding any of the 3rd party stuff.