Pretty sure that said that was a no go. 
Yeah, sorry to disappoint, but at least for now still won’t work. As far as I could tell from the source the 4.17 plugin support indeed allows you to include your own custom shaders. However, there seems to be no way to have these shaders interact with the material editor, which is obviously essential for the toon shading model.
Maybe I missed something, as I haven’t actually found any documentation for the feature yet, but don’t hold your breath just yet
To what extent do you mean? Like is it enough to be able to implement like a shading model in the drop down menu (Default Lit, that area) or no? I’m considering turning another toon model I saw into a plugin if that’s possible, but I know yours requires new pins in the editor, so not sure if that’s the degree to which plugins won’t go.
As far as I could see there is no support for new shading models at all.
If that’s the case, severely disappointed in feature.
Wait, I thought new system would allow people to write plugins that add more shading models to materials Shading Models in Unreal Engine | Unreal Engine 5.1 Documentation. It doesn’t?
Nope. The new system allows plugins and projects to have priority on loading shader files over default engines ones. That is it. To add new shading model, you still need to change at least 3 source files.
I hate to say it, but Unity wins hands down for NPR rendering. It’s such a pain to have to update and build shading model manually in order to get all the bug fixes/features that come with each engine update.
So it’s 2019 now and we still cannot add new shading models to the material editor via plugins or dropping them in a project folder? e.g. https://forums.unrealengine.com/community/work-in-progress/41288-toon-shading-model?70150-Toon-shading-model and https://forums.unrealengine.com/development-discussion/rendering/1409241-abusing-ue4-s-reflection-system-for-cheap-translucency. Why is it still the case that we have to edit engine files and recompile?