alt Hacker NewsThat was because LOADALL was impossible to preserve, since the internal state of the CPU changed in the next models.
80386 also had an undocumented LOADALL instruction, but it was encoded with a different opcode, as it was incompatible with the 80286 LOADALL, by restoring many more registers.
After 1990, no successors to LOADALL were implemented, because Intel introduced the "System Management Mode" instead, which provided similar facilities and much extra.
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.