The OPCustomize is a file that can be used to hide or unhide nodes, in this case, it is used to hide some ancient Redshift nodes that are still visible on the current RS Builds to offer backward compatibility. The good thing is, the OPCustomize can hide the nodes but they can still be loaded from old files, which is a win-win scenario because thanks to the OPCustomize the users can avoid confusion on accidentally using some of those ancient nodes.

Hidden nodes:

• Normal Map → Replaced by Bump Map node with Tangent-Space Normal mode.
• Particle Attribute Lookup → Replaced by RS User Data nodes.
• Vertex Attribute Lookup → Replaced by RS User Data nodes.