Couldn't you deactivate the constraints and removeFromSuperview inside the nextPartialResult closure?
That way you wouldn't need to run through the whole array 3 times.

Something like

@discardableResult
func removeAllArrangedSubviews() -> [UIView] {
        let removedSubviews = arrangedSubviews.reduce([]) { (removedSubviews, subview) -> [UIView] in
            self.removeArrangedSubview(subview)
            NSLayoutConstraint.deactivate(subview.constraints)
            subview.removeFromSuperview()
            return removedSubviews + [subview]
        }
        return removedSubviews
    }

If someone will confuse on how to use magpali code then here's the same:

extension UIStackView {
    @discardableResult func removeAllArrangedSubviews() -> [UIView] {
        let removedSubviews = arrangedSubviews.reduce([]) { (removedSubviews, subview) -> [UIView] in
            self.removeArrangedSubview(subview)
            NSLayoutConstraint.deactivate(subview.constraints)
            subview.removeFromSuperview()
            return removedSubviews + [subview]
        }
        return removedSubviews
    }
}

Or you can do it like this:

stack.arrangedSubviews
            .filter({ $0 is SomeView })
            .forEach({ $0.removeFromSuperview() })

Swift 4/5
https://gist.github.com/yasirmturk/0b47d18a30722902a9a4aaad05d1794a

Please corret me if I'm wrong but from my understanding you could simply do the following to remove all arrangedSubviews:

extension UIStackView {
    func removeAllArrangedSubviews() {
        arrangedSubviews.forEach {
            self.removeArrangedSubview($0)
            NSLayoutConstraint.deactivate($0.constraints)
            $0.removeFromSuperview()
        }
    }
}

Worked like a charm 👍

Just the SnapKit version:

import UIKit
import SnapKit

extension UIStackView {
    
    func removeArrangedSubviews() {
        guard !arrangedSubviews.isEmpty else { return }
        let subviewsToRemove = arrangedSubviews.reduce([]) { allSubviews, subview -> [UIView] in
            self.removeArrangedSubview(subview)
            return allSubviews + [subview]
        }
        subviewsToRemove.forEach { subview in
            subview.snp.removeConstraints()
            subview.removeFromSuperview()
        }
    }
}