Hi.
I just installed ESXi on my CM4, it works no problem.
I used the offiziel I/O board, but it should work with others as long as they expose the "disable eMMC boot" jumper and the slave USB port.
All you need to do is to set the "disable eMMC boot" jumper, which will bring the CM4 up in usb boot mode, after power on; then you can use the "usbboot" tool from the official RPi GitHub to get the CM4 into "mass storage device" mode ()connect the micro.USB slave port with your PC), which lets you see the eMCC as kind of a normal flashdrive on your PC.
Then, whenever a tut or guide tells you to do something with the SD, you just do it with this USB device.
Still need to install the actual ESXi onto a USB drive, though, I don't think it'll be able to be installed and boot from the eMMC - which is kinda sad, but hey, at least it works.