iOS dev here who’s built apps using the force touch feature, in my opinion it never got much traction with developers because the standard ui toolkit elements were never extended to make use of pressure touch. So instead of being able to create UIs by connecting storyboard outlets to function calls like probably 95% of iOS app UI development, a dev would have to drop down to and override the lower level calls to touchesBegan, touchesMoved and touchesEnded functions that govern the phases of a touch where the force pressure information was available, a much more involved and tricky coding and debbugging experience.

But if you were willing to do that, whole new interface possibilities open up because you have an entirely new dimension to work with! I made a music instrument app where position on screen controls note/frequency and pressure controls amplitude and it works very smoothly[0]. But all the established paradigms that devs and users know revolve around 2D, so to take advantage of the force touch you have to both invent a new paradigm and teach it to users, all while supporting devices without 3D touch or losing that share of the market. There weren’t enough takers or it didn’t happen fast enough so Apple decided it wasn’t worth keeping, which is understandable but also kinda tragic in my admitedly biased opinion.

[0] Xalophone digital musical instrument incorporating 3D Touch: https://apps.apple.com/ca/app/xalophone/id1420669553