Using older microcode is just a matter of preventing your OS from uploading newer microcode during the boot process, and not updating the motherboard firmware to a newer version that bundles newer CPU microcode. Rolling back the motherboard firmware is usually not a supported option, and sometimes is actively prohibited by the system.
CPUs do not have non-volatile memory for storing microcode updates. It has to be uploaded from the motherboard during the boot process, and the OS can optionally upload a newer version as part of their boot process. So the difficult thing is finding a matching motherboard that's running an old enough firmware version, or that can be rolled back to an older firmware version.
Socketed BIOS chips used to be somewhat common in gaming boards, which would always allow you to downgrade the firmware. I don't think they're found in workstation or server boards.
If the BIOS EEPROM chip is not a socketed DIP-8 package, often it uses a surface-mount SOIC-8 chip, which can be easily programmed with a SOIC-8 test clip and an I2C/SPI programmer. In case you don't have one, you can also use any single-board computer, like a Raspberry Pi and flashrom for the job.
