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?

I do leave the charging port to headphone adapter dongle plugged in, could this be the source of the drain?

What happens if you remove the adapter for a week or two? Any change?

I just updated to 15.8.4 on my iPhone 7 a few days ago. Now my battery level will drop to 2% even powered off. I only use it for music, it has no sim. I had the battery replaced 2 years ago by Apple. Battery life is at 91%. Before this update it would last for days. I do leave the charging port to headphone adapter dongle plugged in, could this be the source of the drain?

Same here. I have an iPad air 2, bought it last year only for watching and taking notes. It was good on iOS 15.8.3, battery would last up to 2 days. Now after the update, it will only last a day. Last evening i charged up to 89 and after i left it for 8 hours it suddenly dropped to 78. Wonder if apple do this on purpose?! Considering this is an old product.

That seems to have been the source of the drain. It's my understanding there is a tiny DAC chip in the dongle, just don't understand how it would drain the phone down when not in use or powered off? Oh well.