I don't think it could be possible. When you change a state, something more usually happens - like changing variables, writing custom properties etc. It is quite impossible to fully restore it if you delete a state...
I suggest you name them "Unused1", "Unused2" or similar, drag to the corner and forget about them
@ Ben, You may be correct, but on the back end the coders could give us the option to remove them from the tree. Essentially if we delete these workflows or state, they could simply be hidden. Or as I just said in my edit above, give the option to add folders.
BTW, I already do label them unused.