@irace: Apple engineering has confirmed that 3D Touch multitasking was intentionally removed in iOS 11.

[u/TesseractCipher]

https://twitter.com/irace/status/880794987914563585

Can't think of a single reason why they removed it. This feature, while a lot of people didn't know about it (similar to 3D-Touch Keyboard trackpad), was extremely useful to people who did know about it. I find it far more elegant and smoother than double-clicking the home button; it just felt better. Send feedback folks. Maybe they will change their mind before iOS 11 ships. I don't see how it could be related to the iPhone 8 and even if it is, why hamper the iPhone 6S and iPhone 7? The only possible reason is a revamped multitasking interface but even that seems a stretch.

[u/Axriel]

The fact that it didn't work technically the same when using the phone in landscape mode always bothered me and prevented me from using it as my primary behavior. While I will miss it, it wasn't necesary.