According to the records at SIL the documentation for Ancient Greek (to 1453), ISO-639 code grc, has been tagged as type "Historical". This means that the language is dead. Latin lat on the other hand is considered to be "Ancient". Both Latin and Ancient Greek are still taught in schools to kids who get a classic western education.
According to the definition Latin is ancient and consequently it must have gone extinct more then a millenium ago. However, the Roman Catholic Church has continued to use Latin as its language. It maintains a dictionary of Latin modern vocabulary. Surely Latin may be old but it never went extinct.
Ancient Greek does not qualify as ancient because 1453 means less then a millenium. Ancient Greek is taught in school. Books, like the Harry Potter books are translated in Ancient Greek. As far as I understand it, there has not been a similar usage for Ancient Greek as it existed for Latin.
When you are to tag a text using the ISO-639 codes and its definitions, a modern text in Latin or Ancient Greek cannot be tagged. The first issue is that the definitions clearly limit the time when texts are to be considered in a historical or ancient language. The second issue is that in order to write a modern text neologisms are needed and/or existing words with a modern meaning are needed to express modern concepts.
When the definitions preclude the tagging of the modern expressions of Latin or Ancient Greek, it means that either a new code is needed to indicate the modern expression or the defintions of these languages are wrong.
I would argue that when a language has not seen continued use, the modern text is assigned a separate code. It is distinctly different and by tagging it as such, it may be clear to the reader of a text that the understanding of such a text does not reflect the language and the time when it was a living language. I would argue for a separate ISO-639-3 code.
My question is what do you think about this ?