VMware never supported creating a VMFS partition on the ESXi boot volume from the GUI.
If there's free disk space you may be able to create a VMFS partition from the command line.
However, what confuses me is the "Legacy MBR" partition label.
Can you please post the following command's output (replace the DeviceName)
partedUtil getptbl "/vmfs/devices/disks/DeviceName"