There is a very odd thing in the “new” vSphere Web Client that’s mandatory as of ESXi 6.5: when you want to change the Guest OS Version to the recommended one, it’s not in the list.
Recommended:
List:
Hopefully it is related to [WayBack] ESXi Embedded Host Client – Bugs: #12 Getting Warning that client OS does not match what is running.
–jeroen