How to view / edit per-layer XMP/metadata?

You'd have to dive into the Photoshop SDK to see what is supported. I'm sure that it could be coded pretty easily but you'd have to learn Extendscript.

Reading that article It seems to be done via Photoshop scripting  through a Photoshop extension. The one in the that article would be for CS4  a lot has changed since CS4. Extension for CS4 for sure would need to be reprogrammed for CC 2019.  Flash panel support was removed.  Search Adobe exchange the extension for layer metadata support. Layer metadat is not a standard feature of Photoshop.

"For many years Photoshop has supported metadata tagging on a per-document level.  Via the File Info interface or the Metadata panel in Bridge, you can edit things like a document’s copyright info, caption, etc.  Photoshop also automatically adds/modifies certain info (e.g. preserving camera parameters stored in EXIF data; generating and storing a global unique ID for each file).  All this info is applied to the whole document, not to the doc’s constituent pieces.

In Photoshop CS4, however, it’s possible to store metadata on a per-layer basis.  The Photoshop scripting system (which can now be controlled by Flash panels) can read and write any text data (keywords, descriptions, etc.–anything you can think of) for each layer.  Just like other properties that are stored for each layer (visibility, opacity, blending mode, etc.), per-layer metadata moves around with layers themselves.  You could, for example, attach a description to a layer, duplicate the layer, and have the description duplicated as well."