I have installed the iOS 14 on my iPhone 6s plus and the battery drains pretty fast. After analysis I got root cause and solution for battery drain issue.
Root cause of battery drain and temperature high issue due to wifi and bluetooth bug.
Solution:
Below steps need to perform to fix ios 14 battery drain issue on iphone.
1) Reset network settings.
Settings-->General-->Reset-->Reset Network Settings
2) WIFI off
Settiings--> WI-FI--> off
3) Bluetooth off
Settings--> Bluetooth--> off
After performing these steps my battery drain and temperature high issue have been resolved. Battery performance back to normal. Please do from ur end hopefully issue will be fix.