Its possible that IOS 14 is better at reading the battery's health more accurately. My own 8 Plus, which is essentially the same as your X, currently shows 87% on IOS 13. I plan to change the battery, but using my 12 Mini as my daily driver now. You should really just change the battery for £/$50 , like acorntoy said, their is no 5.8" size iphone anymore, so a fresh battery will give it a new lease of life.