They could still preserve it for backwards compatibility in microcode. They didn't do that because the 386 made it possible to get out of protected mode, and in fact allowed a more efficient implementation (big real mode) without undocumented opcodes.

It amuses me that the SMM state save area still lists the descriptor cache fields as reserved, even now that (thanks to virtualization) descriptor caches and big real mode finally have become an official feature of the architecture.