I thought the same, but the internet says that not only does it contain aspartame, but it always has. I feel like I've discovered a new Mandela Effect.

The difference in taste is from what their formulas are based on. Diet Coke basically came from Tab, where Coke Zero was a modern attempt at making Coca-Cola with zero calories.

I’ve been paying more attention to my health lately and honestly going from occasional sugar free soft drinks to even more occasional full sugar drinks has been a great “perk”. The sugary version tastes better, and 300-400 extra calories per month isn’t a big deal when I’m paying attention to my actual day to day consumption.

Besides, yeah yeah natural fallacy etc, but those non-sweetener ingredients probably aren’t stellar for your health either, and less overall volume of this stuff is probably better.