With the introduction of 3D Touch on
iPhone 6S, you were able to do multitasking with 3D Touch by pressing the left
edge of the display and Apple has continued the 3D Touch multitasking feature
upto iOS 10. But now Apple now remove that feature on upcoming iOS 11.

3D Touch multitasking was a simple gesture and was a faster way to switch between apps rather than double pressing the Home button. Now this feature is removed in iOS 11 intentionally.
Bryan Irace tweeted about the removal of
3D Touch Multitasking app switcher;
Apple released iOS 11 beta 2 for
developer’s testers and recently allowed for public beta.
In iOS 10 or older, 3D Touch
multitasking was a simple gesture and was a faster way to switch between apps
rather than double pressing the Home button. Now this feature is removed in iOS
11 intentionally.
Want this feature back in iOS 11 final??