Hey there, CRWDOG,
Thanks for stopping by the Apple Support Communities about your iPhone battery. If I understand, after updating to iOS 13 you've noticed the battery of your iPhone 6s Plus seems to drop much quicker than expected. I'd be happy to help!
There are a lot of factors that go into your battery life and performance. Updating the iOS software, especially moving from one version family to the next (like moving from iOS 12 to iOS 13), the phone will take some time to adjust to new features and settings introduced with that update. During that time, you may experience unexpected changes in the way your battery lasts or charges.
This should only last a short time, though, and if it persist longer than a couple of days, you can check out the information outlined in the Apple Support article iPhone Battery and Performance - Apple Support to get more information on the battery, its capacity, and performance. You can also use About the battery usage on your iPhone, iPad, and iPod touch - Apple Support and Charge and monitor the iPhone battery - Apple Support for more help managing your battery life and performance.
Cheers.