How to undo the "join" command without pressing control or command Z? How can I "un-join" curve objects that were joined with the Join command without having to convert them to mesh objects? If this is not possible, why hasn't it been implemented?
You can separate selected splines into new curve objects.

Go into edit mode &#8633; Tab of the curve object. Select the part which should be separated (linked selection L is useful here). Press P to separate.