Is it impossible to create neumorphic soft UI on Swift?

In order to achieve this the Swift 4 code is no different to Swift 5. So you should just be able to use the code from whatever non-SwiftUI tutorials you've found.

The basics of replicating this though is you need two shadows, one that sits above and to the left of the view, and another that sits below and to the right.

yourView.layer.masksToBounds = false

let cornerRadius: CGFloat = 15
let shadowRadius: CGFloat = 2

let darkShadow = CALayer()
darkShadow.frame = bounds
darkShadow.backgroundColor = backgroundColor?.cgColor
darkShadow.shadowColor = UIColor(red: 0.87, green: 0.89, blue: 0.93, alpha: 1.0).cgColor
darkShadow.cornerRadius = cornerRadius
darkShadow.shadowOffset = CGSize(width: shadowRadius, height: shadowRadius)
darkShadow.shadowOpacity = 1
darkShadow.shadowRadius = shadowRadius
yourView.layer.insertSublayer(darkShadow, at: 0)

let lightShadow = CALayer()
lightShadow.frame = bounds
lightShadow.backgroundColor = backgroundColor?.cgColor
lightShadow.shadowColor = UIColor.white.cgColor
lightShadow.cornerRadius = cornerRadius
lightShadow.shadowOffset = CGSize(width: -shadowRadius, height: -shadowRadius)
lightShadow.shadowOpacity = 1
lightShadow.shadowRadius = shadowRadius
yourView.layer.insertSublayer(lightShadow, at: 0)

There are 3 main things in the above code that are required otherwise the shadows will not render.

1. The view must set masksToBounds to false—since the shadows will be outside of the bounds
2. You must assign a backgroundColor to the layers
3. You must set a shadowOpacity since the default is 0.0—setting 1.0 produces the closest results to your image

You likely want to have the shadows flip when the view is pressed, for that you simply need to switch the shadowOffsets of both the sublayers.

lightShadow.shadowOffset = CGSize(width: shadowRadius, height: shadowRadius)
darkShadow.shadowOffset = CGSize(width: -shadowRadius, height: -shadowRadius)

and when the user stops pressing reset them

lightShadow.shadowOffset = CGSize(width: -shadowRadius, height: -shadowRadius)
darkShadow.shadowOffset = CGSize(width: shadowRadius, height: shadowRadius)

For a UIControl you will likely be doing the above in the didSet of an isHighlighted override.

You can use a pod for adding Neumorphic Soft UI to a UIButton in UIKit such as on: https://github.com/PallavAg/SoftButton