Is wearing modern underwear not from the Sunnah?

Undergarments, like other clothing, are considered customs that are based on people's norms, as long as they do not violate Islamic law. Wearing trousers was known during the time of the Prophet, peace and blessings be upon him. There is no harm in wearing undergarments unless they contain unlawful things like crosses or images of living beings, or if a woman wears what is specific to men and vice-versa.