OPC UA defines a mechanism to identify Nodes in an Information Model as deprecated by using the IsDeprecated ReferenceType pointing from the deprecated Node to some information identifying the version when it has been first deprecated.

In general, the reason to use this feature in an Information Model is to mark some Nodes not to be used anymore, if possible. Often, there are other Nodes in the Information Model to be used instead of. The deprecated Nodes are still available so that Clients developed and configured against an older version of the Information Model are still working. New Clients or new configurations of the old Clients should not use the deprecated Nodes anymore as future versions of the Information Model will not have those Nodes anymore. This allows a smooth transition from the old, deprecated Nodes to the new Nodes without immediately breaking the Clients, and still avoids that a new Client or the configuration of an old Client uses the deprecated Nodes.

From a Client’s perspective, it should avoid using the deprecated Nodes. The only reason to use them is to be compatible with older versions of the Information Model. If this is required, the Client should ideally have two paths to access the Information Model. The default should be the access using the new Nodes. If the new Nodes are not available in older versions of the Information Model, the deprecated Nodes may be used as a backup access path.