Is saying "So-and-so is unofficially married to so-and-so, and they are living together as a couple, contrary to the truth" considered slander (qadhf) against chaste women (muhsanat)?

An 'urfi' marriage, if it fulfills the Sharia (definition) conditions—such as the presence of a guardian (wali), a dowry (sadaq), and witnesses—but is not officially registered, is a valid marriage. Describing a chaste woman (muhsana) as having engaged in such a marriage does not constitute slander (qadhf). However, if the common understanding in the questioner's country considers 'urfi' marriage to be something that tarnishes honor and chastity, then describing a chaste woman as such would be considered slander, because the definition of slander is subject to the customs and terminology prevalent in a given country.