probably not helpful and just one data point, but i thought i'd chime in. after reading all of these widely vary anecodotes, i was a little anxious about upgrading to 14.0, but decided to trying it out thinking if i did it fast enough and had problems, i could revert back to 13.7 if it was miserable battery life before Apple stopped signing 13.7
well, for my 11 Pro, so far, there has not noticeable change in performance. and most interestingly, i didn't notice any real indexing or major battery hit after the upgrade. i have my 256gb 11 Pro about 160gb full, so this was a bit surprising.
after upgrading yesterday, i went to bed with it at 88% and it dropped to 83% after sitting on my counter for 9 hours. that was similar to what i experienced in ios13.
so far today, with normal use, i have seen what is roughly comparable drain using the phone normally. i'm now down to 69% after about an hour of screen time this morning including reconfiguring my home screen and playing with widgets.
i continue to find that battery issues or lack of them after updates are hit and miss for people. i myself have had the bad experiences people have shared here in other iOS updates. it's maddening how inconsistent these are and how some of them aren't even solvable with a new phone set up. i wish those with troubles the best of luck with their issues.
Aside: i got this 11 pro in march with 13.3. i would say that and 13.3.1 gave me the best battery life. i have had a noticeable by tolerable decrease in battery longevity in moving up to 13.7 (and the steps along the way)...not a earth shattering drop, but i would guess at least a 10-20% drop in endurance in the span of those updates. for me, 14.0 seems comparable to 13.7 but that's of course with less than 24 hours of experience with 14.0 so take that with a grain of salt.