I have always assumed that any type derived from an open type was, by definition, open.
However, I recently read the rule for this in CSDL-XML 6.3, Open Entity Type which states:
"An entity type derived from an open entity type MUST indicate that it is also open."
Which could be read as requiring that the XML explicitly specifies OpenType="true" in any type derived from an open type.
The same wording exists in Section 6.9, Open Complex Type, as well as the corresponding sections of the JSON CSDL specification.
While it should always be valid to specify OpenType="true", I would think it would be implied, and not required to be explicitly stated, since the base type is open.
It should be invalid to specify OpenType="false" if the base type is open.