it's now coca cola zero sugar.

Literally everyone I know calls it "Coke Zero" - in fact, the "Zero" pattern has spread to the various soda companies to reflect the particular style of zero sugar.

Sure, that's why it was a weird rebranding.

They probably felt that it stopped being cool, it's now just a common name, so they went ahead and made it into something more legible.

I only just now noticed it changed in the US, which apparently happened in 2022. I remember seeing "Coca-Cola sin azúcar" and "Coca-Cola sans sucre" in foreign markets before, I feel like I saw that pre-2022. I don't recall ever seeing a "Coca-Cola cero", for instance. Kind of feels like it was more aligning the brands internationally but maybe I'm just misremembering.