It appears that the belt slot has gone missing. At least it doesn’t seem to appear in the recent february 2020 blog update in the image of the character shown. Blizzard should add a belt slot, and the belt should have a number of slot like D2. But instead of filling your belt with potions, as in Diablo 2, you attach charms that modify the stats of your character. Those slots will then be subject some minor tetris, as the player weighs pros and cons of different charms. This mechanic will also add some immersion, as you imagine your character donning magic charms, enchanted by the local shaman, or picked off the corpse of one of those vile cannibals!
Belts will have affixes just the same as other items have.
Regardless, as the character is clearly shown wearing a belt, there should be a belt slot. 