It is technically allowed to use EnumStrings/EnumValues on any Variable, as there is no explicit prohibition in the OPC UA specifications.
However, best practices for modeling suggest limiting this behavior to specific cases. (see addtional information)
If a Variable has an Enumeration DataType, the EnumStrings/EnumValues are already provided through the DataType itself and do not need to be redundantly replicated on the Variable (no reference/no new child node property instance).
However, for client convenience and improved accessibility, it may be acceptable for a Variable to have a HasProperty reference to the DataType’s EnumStrings Property.

Discussion Point:
Should it be considered a best practice to:

• Avoid duplicating EnumStrings/EnumValues on the Variable if they are already available via the Enumeration DataType?
• Allow a HasProperty reference to the DataType Property EnumStrings/EnumValues only when there is a clear convenience or usability advantage for clients?

Proposal for Best Practice:

• For Variables with an Enumeration DataType, rely on the EnumStrings/EnumValues provided by the DataType.
• Avoid creating redundant or inconsistent EnumString definitions on Variables, if required reference the EnumStrings/EnumValues from the Enumeration DataType direct..