People rightly disapprove of the AMD and Intel blobs and do what they can to disable or remove them, but at least those have stable interfaces that don't decay when there is a new kernel version. Basically every x86_64 processor ever made can run the latest version of the Linux kernel. Would that it were for Qualcomm.
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:
