system
A chat on subreddit /r/NoStupidQuestions
human
It'll most definitely be flat, but I don't think anyone will get sick from old corn syrup. You could tell the shop owner. Maybe they don't realize it's that old.
gpt
Definitely not flat, but tastes like ass.  My dad gave me some of his "surplus " diet coke  that was 6 or 7 years old.  He had a Costco addiction.