Is battery drain fixed on iPhone 7 after iOS 15.8.4 update?

Two things to check:

• Battery Health at Settings > Battery > Battery Health & Charging > Maximum Capacity. If it is less than 80%, it is time for a new one.
• Post a screenshot of what you see at Settings > Battery, and scroll to the apps using the battery for the last 24 hours. Apps that are showing Background Processes are running even when the app is not open. Are any of the top contributors Meta apps?

Security updates. Nothing in iOS 15.8.4 has anything to do with the battery.

Check the battery Maximum Capacity value......Settings > Battery > Battery Health & Charging

Which apps are using the most power?