It is not allowed to remove any Node that has been defined before. That includes DataTypes, VariableTypes, ObjectTypes as well as standardized Instances or InstanceDeclarations. Servers may not expose all Nodes of an Information Model, if they do not use them, but the Information Model itself has to keep them for backward combability. This does include optional InstanceDeclarations.

This rule avoids that any NodeId or BrowseName in the hierarchy is used in later versions again for a different purpose.