iPhone 5s battery draining when in Airplane AND Sleep AND Low-power modes together, as if I am using it
It's possible that the battery health has degraded over time. I would recommend you install Battery Life in your iPhone to check the battery health.
If the battery health is below 80%, then you should consider replacing the battery. I replaced my iPhone 5's battery after 3-4 years after it hit 65% battery health. You should do the same. I was able to find a 3rd party battery and replace it myself. The process took me 30-40 mins. Also, go to the Battery section in the Settings app and see which apps are the biggest battery users.