unwritten has 3 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is written.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
based on custom rather than documentation
“an unwritten law”
said or done without having been planned or written in advance
“he made a few ad-lib remarks”
Synonyms, antonyms and the broader and narrower relations for unwritten come from Princeton University’s WordNet. Full sources and licences.