MPxTransform_BoundingBox has been added to Maya 2019.2. It has been added as a temporary fix for the issue where MPxTransform::boundingBox() is not called from a command when MPxTransform::treatAsTransform() returns false.
In order for MPxTransform::boundingBox() to be called from commands, you will need to change your MPxTransform-derived classes to MPxTransform_BoundingBox-derived classes.
MPxTransform_BoundingBox will be in subsequent 2019 update APIs, but will be removed in Maya 2020 APIs.
MPxImagePlane no longer needs to be associated with MPxImagePlaneOverride when drawing a custom image plane in VP2.0.
Any code that uses MPxImagePlaneOverride should be updated to use only MPxImagePlane.
The customImagePlane example has been updated to use MPxImagePlane instead of MPxImagePlaneOverride.