A few weeks ago, when iOS 10 came out, I saw an article on TechCrunch highlighting 15 neat hidden features they found in the update. While flipping through their slideshow, I noticed that almost a third of the entries (4 out of 15) were related to 3D Touch.
Did you know you can deep press on the "Send" button in Messages to make your message bigger or add corny confetti effects? Neither did I. Did you know you can deep press on the little "X" button in the Notification Center to clear all notifications? That's a useful one. And the list goes on.
If you ask me, 3D Touch is the greatest UX challenge facing iOS developers right now - and yet I hardly see any apps trying to innovate when it comes to educating users about pressure-based touch features. I find myself furiously hitting UI elements trying to find "hidden" 3D Touch functionality in apps - a frustrating experience in screens that have tons of little components.
So what can iOS UX designers do about this? Well, here are a few suggestions from yours truly. Disclaimer: I am not a UX expert, these are coming from a user perspective.
use animation when displaying the screen for the first time (or maybe always?) to indicate certain things can be force-touched: animate the background color, add a quick "bounce" animation to imply the thing can be "pressed in", and so on;
have some sort of permanent visual cue on elements that can be force-touched; this could be a minor shadow a slightly different background color, a little "handle" to the side of the element, etc;
When the user performs an action that could be simplified by using 3D Touch, display a "Did You Know" informing them there is a possible shortcut for what they just did;
I don't like onboarding tutorials, but they're still an option; this could also be a "Did You Know" sort of thing when launching the app. Those would only be displayed on devices that support 3D Touch;
If the app has a help section, add a "Supported 3D Touch Gestures" guide there;
Get creative - UX designers, you're ingenious folk. Put your thinking/testing caps on and save us!
From those options, I personally think the temporary visual cue (with gentle animation) is the way to go, especially if that becomes a common idiom across apps from multiple developers. The absolute best, of course, would be an official UX solution proposed by Apple and suggested on their extensive Human Interface Guidelines document - I couldn't find a reference to 3D Touch anywhere!
While an official/common solution is unavailable, here's an idea: if your app supports 3D Touch and suffers from discoverability issues, use your social media channels to make short posts about those features and how they can save time. People love time saving life hacks and shortcuts, and you may even get some free shares/tweets out of it.