Then turn it off (Settings, General, Accessibility)

Although lacking discoverability, I personally find it very quick, useful and natural in a couple of use cases: acting on notifications, keyboard cursor movement and selection, (sadly defunct) app switching, on Apple Music track listings.

Oh right, I use the keyboard cursor movement all the time.

Haven't found a single other place where it does anything I find useful.

You're missing out. 3D Touch is pretty powerful and provides access to more than "Useless Options" now.

It still bugs me as very undiscoverable. I don’t want to press things I don’t intend to toggle just to see whats underneath them.

Maybe I’ll get used to it but right now I’m old man grumpy.

sometimes an app shows a popup, sometimes you show the delete icon for all app. wonderful interface.

It always shows a pop-up.

How do I know when I can use it? and if my phone doesn't have 3D Touch, how can I access the same features? (I don't know if my phone has 3D Touch, but I guess it doesn't because I've never been able to make it do something different. Except once when I long clicked the control centre and it did something different. I don't know if that's 3D Touch though. My iPhone used to teach me how to use it, now it just hides things because it makes someone else feel like an elegant designer.)

You don’t, which is one of the two big issues. You can only try it everywhere and try to remember what happens.

(The other problem is the light touch/hard touch thing)