I don't think the Sov ever actually had special armor on it. Ablative armor isn't really a separate metallic layer anyway, it's more of a coating that boils off to dissipate incoming energy. If it's a clear coating, which it seems to be on the defiant since its hull doesn't look substantially different from any other ship, why not just apply it over windows too?
Ablative armor is likely a metal alloy. The X-33 uses (or would have used) a Metallic Thermal Protection system that is designed to dissipate and regulate the amount of Heat that reaches the ship's structure. The shuttle did the same thing. The shuttle's skin was made of aluminum and the frame was made of titanium. Without that protection it quickly disintegrated under the temperature of 2300 degrees Fahrenheit.
In this case we know windows in Trek are merely transparent aluminum. Aluminum has a lower melting point than our strongest metals. The difference between aluminum and Iron is so great in the heat department it would be ridiculous to even venture into the 20 times harder than diamond alloys that Trek makes their ships out of. Essentially even with the a clear coat over the windows it's a foregone conclusion that the windows will fail before the Ablative armor does. The easiest thing is just not to have windows.
Voyager essentially proved that the Ablative armor was not a clear coat.
Defiant's hull doesn't change with the armor simply because of production cost and even if they had the money it wouldn't be anywhere as thick as Voyager's shape altering armor which had to be 2 meters thick in some areas. What ever it was...it has to be some amazing stuff to do what Defiant was able to pull off with a Vorcha parked on the tail pipe with what is apparently only centimeters thick.
Edit:
Considering The Venus probes camera lenses were made of diamond to with stand the great temperature and pressure on the surface I think Scotty gave away a tiny secret. Federation windows have to be made of something else just considering where the Delta Flyer, and voyager have flown through.