6s iPhone.
still love this phone.
but you are correct. i have also started a thread on this issue elsewhere.
iOS 13.1 really killed ability to use the phone in the way that i had been.
was able to get 7 to 8 hours before, but now about 4 to 5 hours.
i can not use the phone in the same ways i was before.
but i have tried literally everything;
no background app refresh; almost all location following turned off. no heavy viewing of videos. etc etc.
still something is eating it.
i have also experiemented with enabling low power mode more or less constantly.
still something is eating battery with this update.
but i also have a macbook air as well. and macOS 10.14.6 battery duration has also taken a stark decrease.
i believe the two issues are caused by the recent updates. i think that they are related.
i think that iOS 13 and macOS Catalina preparation in Mojave's final updates has changed how the mac and the phone are seeking either network or iCloud services.
on the iPhone, carrier signals that were strong before are no longer strong.they are very weak now.
i am hoping that once Catalina comes out, things settle down between these two devices.
again, you and i both share the fact: iPhone 6s ran well using iOS 12 all versions.
we pray that the 2020 spring iPhone rumored to be an iPhone 8 replacement is worthy.