There’s been no change to LR’s handling of custom XMP since version 1 – I’d characterise it as “preserving” custom XMP data.
So LR does read the data from imported files and stores it in a database field. It never displays the information and it isn’t editable even through the SDK. But at least Lightroom doesn’t do any damage. When you save metadata back to the original files (sidecars or directly into the file) it does not overwrite any custom XMP it encounters, and it does write the custom XMP to any exported files.
It is possible to use a plug-in that calls an external library (exiftools) to read and write custom XMP. I have one working for myself, but I judged the area’s complexity and potential for damage to be so great that I’ve chosen not to develop it for wider use. I’d make far too many enemies!