Finding the perfect word to describe something that is not modern can be tricky. There are many synonyms that can be used to replace the phrase "not modern." The word "archaic" is a great option, as it suggests something that is old-fashioned and no longer in use. "Antiquated" is another option, which refers to something that is outdated and obsolete. "Traditional" is also a useful choice that conveys a sense of something being old-fashioned, but still holding value. Additionally, "outdated," "old-school," and "obsolete" are all good synonyms to describe anything that is no longer relevant in today's society. Ultimately, the synonym chosen will depend on the context in which it is used and the exact meaning desired.