Battery of 6s draining

Hello, I have a (not even) two year old iPhone 6s. Costed me a bundle at the time. The battery has always been sub par, so I've always followed all the battery saving tips apple offers. All apps closed, GPS off, etc. Lately, however, the battery has become intolerable. I charged it to full 4 hours ago and it has been on airplane mode in my pocket for the duration of my train journey up until this point. I took it out of my pocket and it is on 20%. Why is this happening, and how is this not being covered by Apple? According to the serial number check my phone doesn't fall into the range with faulty batteries, I shudder to think how bad those must be!