[kens]

The microcode is protected by copyright; see NEC Corporation v. Intel Corporation.

[dlcarrier]

Microcode is specific to a given implementation, so if you make your own x86 implementation, it's not going to run AMD's or Intel's microcode unless you go out of your way to make it do so. NEC didn't infringe Intel's copyright, because their processor ran different microcode than Intel's, and NEC won that lawsuit.

[sylware]

Allright, the ISA itself is probably protected under some copyrights(+patents), which in the US last for ~one century (cf Disney).

In the end, nobody sane would try its luck, better go for something non "IP-locked".

Aka RISC-V, not to mention that for a modern implementation RISC-V is more friendly.