As shared back in 2022 in my Homelab Considerations for vSphere 8.x blog post, if you attempt to install ESXi on system where the CPU is no longer being supported by the OEM vendor, who then informs VMware on what to publish on the VMware Hardware Compatibility Guide (VCG), you will see the following error message preventing you from proceeding.
Also mentioned in that article, you can override this default behavior with the following ESXi boot option: allowLegacyCPU=true which would convert this from an error to warning message but allowing you to proceed in an installation or upgrade.
Obviously, this is not officially supported by VMware and its behavior was never guarantee but it certainly was a nice gesture, in my humble opinion from Engineering, as they could have easily not allowed any override based on on our official VCG. For homelab use cases, it might be okay to use a CPU that is 5 to 6 years old, knowing that it should be replaced hopefully in the near future.
I just had a couple of users report issues while upgrading to the latest ESXi 8.0 Update 2 release when using the allowLegacyCPU boot option and saw the following error message
VMB: 716: Unsupported CPU
The ESXi installer immediately halts and prevents the upgrade from proceeding ...