I don't want to come across as pessimistic, but I seriously doubt anyone from Apple is ever going to read this thread. They just don't care. I haven't heard back from the Senior Apple Support person in four days that is handling my case, despite multiple emails.
My suggestion is to call Apple and start hounding them. Odds are they're not going to do anything to go out of their way to help. That's just their [superior] mentality. Basically we're all hosed. iOS 13 dorked up our phones and we're stuck with it.
They'll tell you to run diagnostics, possibly multiple times. They won't find anything wrong. They'll suggest you go and replace the battery for $50 with no discount, and it won't make a difference. Some of you already have had a very recent battery replacement and are having the same problem as persons with older batteries. They might even ask you to go into a store so a technician can do "more diagnostics that we are not capable of performing remotely.". Guess what the in-store person will find wrong? Nothing. Will the take detailed notes and add them to your case? No.
Once you've wasted your time at the store and call them back (which the person in the store will suggest you do), then they're going to tell you to reset your phone's settings. Not necessarily wipe the phone, but reset all of your settings (they might suggest just the network settings first).
Good luck, cause you're gonna spend the next hour or two reconfiguring your phone as to how you had it set up. Still won't make a bit of difference. Once you get past that point, and show there is no improvement they'll ask you do to do a complete phone reset, back to the way it came when it was new, but with iOS 13 on it. Will it help? Nope. Just a colossal waste of your time. After each of these steps you'll be having to call Apple back and wait again to speak to someone.
I strongly encourage that a backup of your phone is made before you let Apple talk you into making ANY changes or performing any tasks. Whether you backup all of your settings to iCloud, or to iTunes (pre-Catalina for Mac owners), or to Finder for Catalina OS. Don't forget to encrypt that backup (if local) for security sake. Take screen captures as you go showing the battery degradation as it affects your device. I did this, to show how quickly the battery was draining. The digital clock at the top of the screen couldn't show enough detail for how quickly it was draining. I actually used the Stopwatch app and took a screen cap each time the battery % dropped, doing my best to react as quickly as possible. Apple didn't care that I did this, however I have hard evidence they can't refute. This will anger them because of course they're smarter than us.
You'll spend hours going back and forth with Apple, keeping in mind you're the one having to do all the tedious work, not them. They're getting paid to tell you do to do the work. You're not getting paid to redo your phones. Someone earlier in this thread mentioned class-action lawsuit. I have no idea how to approach that, but I'd be a willing participant.
At some point you'll have to ask yourselves, "how valuable is my time to keep messing with this brick?" Apple already has your money, they're not interested in solving the problems they have created. They just want you to spend more of your money on a new (presumably) iPhone, and won't give you a discount on it, even if you totally prove the OS is the culprit. Remember, there is only one variable here, the OS. Your phone and its hardware didn't magically change, just the upgrade to the operating system. Apple hates hearing that. It means that you are suggesting they have an inferior product.
What's worse is that you can't even sell the phone to offset the cost of a new(er) model. You have to disclose that it has a flavor of iOS 13 on it, and the integrity of the battery has been seriously compromised. The phones are now basically worthless.
If and when you reach someone at Apple support, ask them for an email address that you can reply to. It's one thing to have a discussion over the phone where they can change the words that come out of your mouth. It's another thing when you have an email chain that can be audited and is a historical record. When the conversation starts, copy the URL of this thread and send it back to them. Hard for them to argue your case when so many people have the exact same problem stemming from the exact same action (upgrading the OS).
Thanks Apple.