No, katanas do not have to be folded. While traditional Japanese swordsmiths historically employed folding techniques in the forging process of katana blades, it is not a mandatory practice for creating a functional or high-quality katana today.

The folding process, which involves repeatedly layering and forging the steel, was traditionally used to refine the material, remove impurities, and create distinct grain patterns. It was a technique employed to optimize the quality and aesthetics of the blade.

Modern advancements in steel production and heat treatment techniques have made it possible to create high-quality katana blades without extensive folding. Many modern swordsmiths use mono-steel or billet steel, which involves using a single piece of steel or combining multiple layers without extensive folding.

While folding can contribute to the aesthetic appeal of a katana, it is not necessary for the blade's functionality or performance. The overall quality of a katana is influenced by various factors, including the choice of steel, heat treatment, blade geometry, and the skill of the swordsmith.

It's important to note that if a katana is advertised as folded or Damascus, it's essential to ensure that it is produced by a reputable manufacturer or experienced bladesmith who follows proper forging techniques and quality standards. The focus should be on the craftsmanship, overall construction, and suitability for its intended use rather than solely on the presence or absence of folding.