These pointers point to the new nodes when duplicating,
and their even used to point to "original" nodes for
"localized" trees. They're just a bad design decision
that make code confusing and buggy.
Instead, node copy functions now optionally add to a map
of old to new socket pointers. The case where the compositor
abused these pointers as "original" pointers are handled
by looking up the string node names.
Differential Revision: https://developer.blender.org/D13518
fdc4a1a590