Those undocumented instructions don't exist on x86_64, and they could be used to do bizarre things on the processors which had them:

> As the two LOADALL instructions were never documented and do not exist on later processors, the opcodes were reused in the AMD64 architecture.[8] The opcode for the 286 LOADALL instruction, 0F05, became the AMD64 instruction SYSCALL; the 386 LOADALL instruction, 0F07, became the SYSRET instruction. These definitions were cemented even on Intel CPUs with the introduction of the Intel 64 implementation of AMD64.[9]

[snip]

> Because LOADALL did not perform any checks on the validity of the data loaded into processor registers, it was possible to load a processor state that could not be normally entered, such as using real mode (PE=0) together with paging (PG=1) on 386-class CPUs.[7]