Quoted from YeOldPinPlayer:
Up until 1964 the playfield glass wasn’t tempered according to Wayne Neyens.
Pg 162 The Pinball Compendium 1930s-1960s, by Michael Shalhoub
I remember reading that somewhere also. Plate glass really wasn't all that bad on older machines because flippers were not as strong anyway. Not saying it was a great idea just saying it didn't matter as much. Most people would be surprised how much plate glass is around them everyday. The one that amazes me the most is the thin glass in storm doors is typically plate (or at least it was until just a few years ago).
Modern pinball machines should never have plate glass in them but logic says lots of other stuff shouldn't use plate glass either!