Given that the Apache Foundation suggests either (their standard header or the SPDX identifier) be used http://www.apache.org/foundation/license-faq.html#Apply-My-Software and the FSF supports generally the use of SPDX identifiers (not in the explicit way as ASF, but this is the last statement I know of: https://www.fsf.org/blogs/rms/rms-article-for-claritys-sake-please-dont-say-licensed-under-gnu-gpl-2 - It seems more appropriate for SPDX to simply not make any recommendation either way related to using standard headers. At least that was the original question/point!