Intel microcode is signed so you can't run open source microcode even if you were able to create it, and the microcode is encrypted so you can't reverse engineer it anyway.
On long obsolete AMD K8 CPUs, there was some work on reverse engineering the microcode back in 2017:
On long obsolete AMD K8 CPUs, there was some work on reverse engineering the microcode back in 2017:
https://github.com/RUB-SysSec/Microcode