So let us assume I have a 3D model of a window and I would like to stretch it like a 9 sliced sprite. How would you go by doing 9 slice on meshes? All I found online was about quads. What about more complicated meshes?
As far as I know, this is not as popular as 9-slicing images, so it doesn't come with popular engines.
Having said that, you can always do this manually. Editing a mesh is easier than editing an image, so it's not difficult to separate a mesh into 9 pieces (like you would an image) and either scale the in-between meshes, or tile them, or a combination of both. You don't have to mess with shaders or anything complicated, just the basic transformations of a 3D object are enough.
The process is the same as when using an image, and maybe you can do a "27-slice" if you want the same effect on 3D.