My iPhone 11 Pro Max’s battery life has drastically declined after the iOS 14.5 update.

iPhone 11 and later models, battery performance --> About the battery and performance of iPhone 11 and later

Protect & Prolong life of your iPhone's Battery:

1. About the battery usage on your iPhone, iPad, and iPod touch ...
2. Batteries - Maximising Performance - Apple (IN)
3. iPhone Battery and Performance

Try some of these options too.

1. Avoid Screen Mirroring
2. Put on Low Power Mode
3. Put Auto-Lock on (1 min):- with low power mode it will be 30 Sec
4. Put Auto Answer Off
5. Disable Push notifications for apps you do not use often
6. Keep your iPhone Facedown to prevent notifications to light up the screen
7. Enable Auto-Brightness:- Settings > General > Accessibility > Display Accommodations > Auto-Brightness [Enable]
8. Manually lower brightness from Control Center
9. If you Want to watch continuous Video. Stream it on TV or a Monitor
10. Don't force close apps; Apps which are not on frontend screen are paused by the system to save energy. Apps require more energy to re-launch. Apps which are open in the background use large memory is a myth and quitting them is counterproductive
11. Location services to be allowed only while using the app
12. Set your iPhone to fetch email and calendar data from internet less frequently
13. Turning off Background App Refresh 
14. Charge before it drops below 20-30%
15. Try connecting to trusted Wifi wherever available instead of Cellular
16. Enable Airplane mode in areas with bad cellular and wifi coverage. Your phone keeps trying for reconnection with the missing/weak network. This uses a lot of energy
17. Switch off Wifi & Bluetooth while in Roaming or Not in Range
18. Enable "Wifi Assist" : Settings > Cellular or Settings > Mobile Data > Wi-Fi Assist [Enable]
19. Periodically Check the Battery Health (Settings > Battery > Battery Health)

iOS 14.6 removed the battery calibration function which was active since installing iOS 14.5. The fact this was only active for iPhone 11 users seems strange to me because my battery was amazing before that and at 100 percent. I believe Apple is throttling our batteries to make iPhone 12 series seem more attractive to buyers.

My XR had a two day battery life: since the iOS 14.5; 20% left after 12 hours. Also looking for the fix!

This is ridiculous. My phone was brand new and working perfectly before ios14.5. Im not adjusting anything, apple needs to stop coercing people to upgrade via sabotage..

My 12 pro needs recharging every 5 hours after 14.6 download. Apple needs to release a fix like yesterday

I have same issue on Iphine XS Max.