I updated the iOS and now 3D Touch doesn’t work

Change 3D or Haptic Touch sensitivity on your iPhone - Apple Support

The above link says......These devices support Haptic Touch: iPhone 11, iPhone 11 Pro, iPhone 11 Pro Max, and iPhone XR. These devices support 3D Touch: iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, iPhone X, iPhone XS, and iPhone XS Max.

So check your settings......How to turn on 3D or Haptic Touch

To turn on 3D or Haptic Touch, and adjust the sensitivity, follow these steps:

1. Go to Settings and select Accessibility.
2. Tap Touch, then tap 3D & Haptic Touch. Depending on the device you have, you might see 3D Touch or Haptic Touch only.*
3. Turn on the feature, then use the slider to select a sensitivity level. 

Haptic Touch is multitouch made multifunctional. With it, you can long-press to launch actions instead of apps, reply to notifications, preview messages and links, switch keyboards, switch apps, animate Live Photos, and more. It's like a wormhole through iOS that lets you do more without having to do more.

Haptic touch replaces 3D Touch in iOS 13 and the iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max. Most of the functions are the same, but how you get to those functions is slightly different. You no longer "deep press" or "hard press" to trigger an action. Instead, place your finger on what you want to get more info or perform a new action and rest it there for two seconds. Then, tap the action you want to use.

So might not work ok on your iPhone 6s

Good question...according to the info I sent...it may be wrong...ask Apple Support...

Click "Contact Support" top right corner of this page