Per several of Joe Strapac's Southern Pacific Historic Diesels series of books, the cost of buying FRA " 223 " bullet resistant glass was too high to continue with the L - Shaped engineer's windshield. Some units were hastily and crudely modified to meet FRA deadlines, with an unpainted metal splice welded into the gap. I don't think crash protection per se was the reason for the change, as in any wreck that crushed the nose enough to contact the cab front glass, it was likely to be punched in from the rubber moldings into the interior anyways.
The 3 EMD SD-45 demonstrators also had L - Shaped engineer's glass too. They were sold 2nd hand to Delaware & Hudson as their only EMD units ( at the time ), then traded to Erie Lackawanna for 3 U-33-C's, which D & H was more familiar with. The L - Shaped windshields did make it into E-L colors; I don't know if they survived into Conrail colors though.