As of Mustang b55.
All platforms.
Attached standalone Java code for an easy replay.
The javadoc for interface javax.management.Descriptor says about the field mxbean:

[...]
The string "true" or "false" according as this MBean is an MXBean. A Standard MBean or MXBean registered directly with the MBean Server or created using the StandardMBean class will have this field in its MBeanInfo Descriptor.
[...]

In case of a standard mbean, the field mxbean=false does not appear into the MBeanInfo Descriptor.