Currently, the description of the indexRange define that the indexRange is not used if the specified Attributes not an array
(https://reference.opcfoundation.org/Core/Part4/v105/docs/7.29#_Ref133162567).
In the NumericRange definition also allows using it for ByteString and String (https://reference.opcfoundation.org/Core/Part4/v105/docs/7.27).

The Device Integration give a hint that ByteStrings can handle similar as arrays, so I assume that the indexRange is also used for ByteString/String.

For a better readability, I think a hint at the ReadValueId/WriteValue that the indexRange works with ByteString/String is very useful. Otherwise, a reader could think that NumericRange can use for Substring, but in this special case it is not allowed by because it is explicit forbidden.