I suggest implementing a transparency effect for objects in a given layer, and also implement such masks which exist in other drawing programs such as Inkscape.
Draw is not Inkscape, but let's ask UX.
It is not clear, what such a feature is proposed to do. Transparency is a property of the individual shape. The layer in Draw are tools to group shapes to include/exclude a group of shapes from printing and/or viewing. Layers in Draw should not be confused with Layers in a raster-graphic image processor.
We're replacing our use of the 'ux-advise' component with a keyword: Component -> LibreOffice Add Keyword: needsUXEval [NinjaEdit]
As Regina said, ODF define layers very clearly as just a grouping feature. If you want a transparency "layer" you should fill it completely with a shape and apply the transparency for this object. The enhancement request WONTFIX because we follow the ODF specification.