Waist
“Before becoming obsolete due to advancements in tailoring, belts such as this were used for holding up trousers and the like.
— In-game Item Descriptions
Removal
With the release of Endwalker, the waist slot was removed. Belts are no longer possible to acquire, and any existing belts can no longer be equipped. This is due to the fact that belts do not have any visible appearance on the player character model (since Patch 2.0), they have low impact in stats, they are not exciting to loot, and they take up the same amount of data as any other gear.
With the data (Specifically, armory slots) freed up from the removal of belts, 15 slots were added for weapons, 15 for rings, and 5 are reserved for later use.
Because belts were removed in patch 6.0, they were not subject to the concurrent stat squish. Therefore, the belt stats shown will appear to be higher compared to equivalent item level gear in the current version of the game.