D.5.2 DataTypeDictionaryType ( https://reference.opcfoundation.org/v104/Core/docs/Part5/D.5.2/ ):
"...
The Deprecated Property is used to indicate that all of the DataType definitions represented by the DataTypeDictionaryType are available through a DataTypeDefinition Attribute. Servers that provide DataType definitions as a DataTypeDefinition Attribute and through a DataTypeDictionaryType shall expose this Property.
..."

Assumption1: The value (TRUE/FALSE) of Boolean property 'Deprecated' is NOT relevant, only the presence of the property itself, or?

(Further assuming that only the presence of property 'Deprecated' is relevant but not the value)

Assumption2: If one (custom) DataType does not provide data type information by 'DataTypeDefinition' property --> 'Deprecated' property is NOT present at corresponding 'DataTypeDictionaryType'; therefore e.g. Clients require data type information from corresponding DataTypeDictionary;

Assumption3: If !!ALL!! (custom) DataTypes do provide data type information by 'DataTypeDefinition' property --> 'Deprecated' property shall be present at corresponding 'DataTypeDictionaryType'; therefore e.g. Clients (supporting 'DataTypeDefinition' property) can "skip" data type information from corresponding DataTypeDictionary and expect that data type information about (all custom) DataTypes are provided via 'DataTypeDefinition' property by server;

--> Are those three "interpretations"/assumptions correct?

If so, the second sentence in description "Servers that provide DataType definitions as a DataTypeDefinition Attribute and through a DataTypeDictionaryType shall expose this Property." should be ommitted as it could be misinterpreted as 'If server provides both (new simplified approach with 'DataTypeDefinition' AND old-style "DataTypeDictionaryType" ) ways the 'Deprecated' property needs to exist (even if at least one custom DataType definitions is not provided via DataTypeDefinition attribute).
But if I understood it correctly the 'Deprecated' property needs also to exist in case ONLY new simplified approach with DataTypeDefinition attribute is used.

https://reference.opcfoundation.org/v104/Core/docs/Part5/D.1/
"...
Annex D (normative) DataTypeDictionary
D.1 Overview
This annex defines a way to provide encoding information for custom DataTypes. In previous releases of the specification this approach was defined in OPC 10000-3. In OPC 10000-3 a simplified approach is now defined having a DataTypeDefinition Attribute on the DataType Node. The approach using DataTypeDictionaries is provided for backwards compatibility and in case some specific requirements cannot be fulfilled with the simplified approach. It is recommended to only use the approach using the DataTypeDefinition Attribute.
..."