When did housing design change to eliminate most interior doors, other than on baths and bedrooms?

Pretty much about the same time that Central A/C and heating system came into place. In the old days you only heated or cooled the rooms you needed with space heaters or window A/C units. The Central units needed to circulate the air in all the house...so doors that were closed caused problems. Most of this happened in the late 40's early 50's.When did housing design change to eliminate most interior doors, other than on baths and bedrooms?
Dunno, but on HGTV, it seems that everyone wants an ';open floorplan,'; where everything is basically in the same room- kitchen, dining room, living room.