The Lost Radiants are commonly believed to have betrayed humankind on the Day of Recreance. It was revealed in Oathbringer that this happened as a result of the Knights Radiant having learned that their ancestors were the ones that invaded Roshar and enslaved its natives (i.e., the Parshendi), which happened while Honor was incapable of inspiring them to fight on as he had done previously. They abandoned their Shardblades and Shardplate, which still seems incomprehensible to people of the modern era.
They are considered to be traitors.
It has been speculated that the Day of Recreance was actually the day when Odium vanquished Honor, thereby Splintering him. This potentially caused all Knights Radiant to lose the Connections they'd had through their spren. Believing that they'd been abandoned, it's possible that the Radiants lost all hope and abandoned their duties as a result.