Last update for my iPhone now kills my battery

Batteries are meant to degrade over time, which means after a while they won't hold the same battery capacity of which they used to.

Try going into Settings > Battery > Battery Health > What % are you on? Anything below 80% is a cause of concern. Apple recommend it should take around two years of usage to reach that level

You might also find this helpful as it provides you with useful information on maximising your battery.

You are unable to downgrade to a previous version of iOS if you are running iOS 14.6, some users have been experiencing similar issues. Apple will likely look into this and offer a fix in a future update.