You are not alone! My battery level has also completely plummeted after this update. I've gone through every setting on my phone and turned off lots of features as well as updating all my apps in hopes that it would help, but nothing has.
Previous to update 17 I had excellent battery life; I'm using a 2nd Gen SE - it is an older model but the phone itself was nearly new. Now since the update I'm suddenly being told that my battery needs replaced.
Supposedly after any update there are background actions occurring that can affect battery life for awhile, but after a month nothing has gotten better.
Then of course they released 17.1.2, which only addressed some security issues, and next month another big update will be released, so who's to say how long we'll have to wait after it to see if it helps?
I really hope that they finally do something to fix this battery drain, but I'm not holding my breath.
Apple has a history of intentionally destroying battery life in an attempt to force users to upgrade, they've even been sued for it by entire countries and found to be liable.
But apparently that hasn't stopped them from continuing to engage in bad business practices that are only going to result in a lack of trust from their customers.
After this poor experience I don't think I'll buy anymore Apple products because there's no guarantee that they won't purposely make my device obsolete or refuse to fix update issues like this in a reasonable amount of time.