Why the dnd module is not supported?
According to the docs it will be replaced with a new DND:
tkinter.dnd
Drag-and-drop support for tkinter. This is experimental and should become deprecated when it is replaced with the Tk DND.
The concept should remain the same, but a lot could change, and I would not be terribly surprised if this happens.
This is very preliminary. I currently only support dnd within oneapplication, between different windows (or within the same window).
As for:
Is this project strictly related to tkinter GUIs development?
I guess, depends what you mean, there are bugs about this in the python issue tracker, and this project is related to creating GUIs, so unless I am not understanding correctly, yes it is.