But if a programming language is not used by any human anymore, is it still alive?

the language can be preserved and it will stay in books and documents sort of like the languages of fiction books you can learn the language and speak it but the language is really dead gone and so are many languages in Brazil, Papau New Guinea and other countries in Africa. Language are born, the grow the die so do cultures. This is the last generation of Tribal existence on every major continent