Whether you are applying a CPU microcode update through a hardware vendor's BIOS update or an ESXi update/upgrade, verifying the CPU microcode revision within ESXi is not as easy as it could be.
Today, if you wanted to get the current CPU microcode revision from ESXi, you would need to enable SSH and then use the vsish interface to retrieve this information by running the following command:
vsish -e cat /hardware/cpu/cpuList/0 | grep "microcode" -A2
While you can retrieve the CPU microcode revision, the method and required interfaces was less than ideal, especially at scale. Given ESXi already exposes a number CPU attributes via the ESXCLI interface, which includes programmatic access when using PowerCLI, I thought it would be a nice feature to also include the CPU microcode revision as part of the output and had filed an internal feature enhancement a few years back.