and as these are Xserve 3,1 are on the HCL for ESXi 5.5. Therefore I can legally accept the Apple EULA, and install Apple macOS Virtual Machines on them, because they have an Apple badge (Apple hardware).
If you do not have an Apple badge on your hardware, you are in breach of the Apple EULA. (Please do not ask me, because this cannot be discussed on Experts Exchange)
If after applying VMware vSphere (ESXi) security update - ESXi-6.7.0-20190504001-standard (Build 13644319) - https://kb.vmware.com/s/article/55636 and adding VMkernel.Boot.hyperthreadingMitigation = TRUE After restarting ESXi you notice half the number of logical processors (e.g. hyperthreading disabled!) - this is NORMAL!. "Enabling this option will result in the vSphere UI reporting only a single logical processor per physical core; halving the number of logical processors if Hyperthreading was previously enabled. In addition Hyperthreading may be reported as 'Disabled' in various configuration tabs."
BEFORE UPDATE VMkernel.Boot.hyperthreadingMitigation = false
AFTER UPDATE VMkernel.Boot.hyperthreadingMitigation = true