I accidentally spilt some coke on the left side of the keyboard 2 months ago. I quickly turned it upside down and tried my best to clean the coke externally. It works perfectly well after that until now, no sign of liquid damage except for some sticky keys. Does it mean that the coke didn't get into the logic board under the keyboard? Is there any possibility that my MBA just dies up one day without any signs? I'm praying hard now
Yes, it is possible that it will die one day.
If liquid went inside the laptop and onto the logic board (or IO board, SSD, etc...), what is left of the Coke could be slowly corroding the components on it.
You can take the laptop apart and look to see where the liquid went. After you take out the logic board, it should be easy to identify where the Coke went and if any components need to be cleaned. You can clean them with a soft toothbrush and 99% isopropyl alcohol.
You can Google for macbook logic board corrosion to learn more.
If you feel comfortable opening your laptop, I think it is best to inspect for damage and clean the components.
If you do not feel comfortable, you could break something and/or lose your warranty even if there is no damage, so it may not be worth it until it stops working. At that point, you may need to replace the part instead, as cleaning may not help anymore.