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.
Take a rendernode as source and a GskPath and GskStroke,
and fill the area that is covered when stroking the path
with the given stroke parameters, like cairo_stroke() would.
