Actually, they didn't change very much and were used in production vehicles through 1999. The big block is still in production for the 454 HO and 502 crate engines. There is also a 572 CID race motor in production that is a tall deck block.
The differences in them are a 1 piece rear main seal, priority oiling, and the heads are different on the gen 5 and 6 blocks. The other changes are pretty subtle. The oil pan is a one piece seal, and the front cover went to a cast aluminum unit.
So, to correct you, if I may: They did not go out of production at any time from the time the big block was put into the first cars/trucks.