The new renderers don't support them due to the required complexity of
integrating them with Vulkan and the assumptions those nodes make about
the renderer (the GL renderer exports its internal APIs into the
GLShader).

There haven't been any complaints that I'm aware of since 4.14 was
released where the default renderer does not support the nodes, so usage
in public seems to be close to nonexistant.

The 2 uses I know of were workarounds about missing features in GTK that
have stopped since GTK now supports them:

1. GStreamer used in to do premultiplication when the old GL renderer
   did not do so in hardware but on the CPU.
2. Adwaita used it for masking before the mask node wa added in 4.10.