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Are you sure the music app does no longer utilize the 3D touch functionality? They afaik only deprecated a few APIs but I don‘t think the pressure detection is one of them.
I have XS Max. Tested Seaboard 5D app on girlfriends XR to see how well app worked with haptic only, and was very disappointed. Super sucked by comparison. So what I know know is:
A) a “Pro” without real 3D pressure is not for me.
B) afraid to upgrade to iOS 13 if it breaks/disables the real pressure hardware on third party apps that need it, like the Seaboard 5d (that I use daily and LOVE... it’s the whole reason I got the XS Max: I wanted 3D pressure and largest screen possible for the S5D app)
 
Apparently you can't do it on FaceID phones. So that's why.
I believe the phones without a home button have some gestures available to switch to the last app or bring up multitasking, so it's possible the 3D Touch version of that isn't there (and perhaps wasn't even before iOS 13).
 
As mentioned before, aside from the peek functionality 3D Touch still works on iOS 13 GM. What I’ve noticed though is that on the lock screen less force is needed to activate the flashlight or camera. Yesterday when I pulled my phone from my pocket my flashlight was on, so that’s a bit worrying.

I have XS Max. Tested Seaboard 5D app on girlfriends XR to see how well app worked with haptic only, and was very disappointed. Super sucked by comparison. So what I know know is:
A) a “Pro” without real 3D pressure is not for me.
B) afraid to upgrade to iOS 13 if it breaks/disables the real pressure hardware on third party apps that need it, like the Seaboard 5d (that I use daily and LOVE... it’s the whole reason I got the XS Max: I wanted 3D pressure and largest screen possible for the S5D app)
I’ve installed the app on my iPhone XS running iOS 13 GM and the 3D pressure still works. The only thing that doesn’t work is when I try to play chords or hold 3 notes the shortcut menu appears, but maybe an update to the app is needed. I’ve also installed it on my iPad Pro (which doesn’t have 3D Touch of course) and indeed it’s different than on iPhone. Curiously chords work here, although it’s on iPadOS 13.1.
 
Working for the same for me as it did in 13.0.

Also, side note, EverythingApplePro did mention it’s faster on 13 than it was on 12. It’s a slight difference but didn’t realize it until he pointed it out. Still, I miss half of what 3D Touch once was.
 
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