3D Touch was a big UX problem, there was never a way to show the user that you could do it and where you could. Apple has a problem with discoverability in quite a lot of features. There's many small little things in iOS that users don't know about and likely will never find out about, perhaps because they want to keep the UI clean.
Why do they refuse to give you a manual? Why are there no instructions. Drives me crazy.
There’s a whole Apple app - Tips - included with every iOS device, dedicated specifically to these small features.
It’s updated by Apple with new tips and tricks regularly, and you can choose to be notified each time a new ‘episode’ is released.