That's a 3E-ism as far as I know.
In the AD&D DDG the Plane of Shadow was a shadow of the Prime Material Plane, cast by the interaction of Positive and Negative Material Planes with the Prime.
In the AD&D MoP the Plane of Shadow had become a demiplane.
I don't know about 2nd ed AD&D/Planescape, but I don't think it really had the idea of "breaching cosmologies", did it?
What is your point (aside from the obvious)?