[Aerbil313]

You can hold down on every key, not just the space bar.

[c-hendricks]

Except for keys that have alternate accent keys, and I don't think it's possible at all on the other keys when you have swipe typing available.

[teolandon]

I think you can only 3D touch on other keys. If you have one of the newer iPhones without 3D touch you have to long press the spacebar.