Layers were released in 15.1 and has been optimized a lot during the Sparx client 16.0 lifecycle, the same time PCS 5.0 was released
It is natural the webservice is a bit behind in features so lets hope it will arrive soon.
Currently the layer function does not functions as initially advertised:
Diagram Layers will be a welcomed tool for people who need to present complex models to an audience, creating an easy-to-understand narrative built up in stages rather than confusing the audience with the complete diagram from the start.
There is currently no method to add an element to multiple layers which would be necessary in many cases to display the narrative built up in stages, the workaround being filters.
So we might expect more development or even an overhaul on the layers feature
That being said, filters are currently not available in PCS neither
It would help if you made a feature request.