SwiftUI ViewModifier to keep track of the offset in a ScrollView. Can be expanded for all UIScrollViewDelegate methods and (x, y) offset.

ScrollViewOffsetModifier.swift

// https://github.com/siteline/SwiftUI-Introspect
import Introspect

struct ScrollViewOffsetModifier: ViewModifier {
    
    @Binding
    var scrollViewOffset: CGFloat
    
    private let scrollViewDelegate: ScrollViewDelegate?
    
    init(scrollViewOffset: Binding<CGFloat>) {
        self._scrollViewOffset = scrollViewOffset
        self.scrollViewDelegate = ScrollViewDelegate()
        self.scrollViewDelegate?.parent = self
    }
    
    func body(content: Content) -> some View {
        content.introspectScrollView { scrollView in
            scrollView.delegate = scrollViewDelegate
        }
    }
    
    private class ScrollViewDelegate: NSObject, UIScrollViewDelegate {
        
        var parent: ScrollViewOffsetModifier?
        
        func scrollViewDidScroll(_ scrollView: UIScrollView) {
            parent?.scrollViewOffset = scrollView.contentOffset.y
        }
    }
}

extension View {
    func scrollViewOffset(_ scrollViewOffset: Binding<CGFloat>) -> some View {
        self.modifier(ScrollViewOffsetModifier(scrollViewOffset: scrollViewOffset))
    }
}

// Example

struct ContentView: View {

    @State
    private var scrollViewOffset: CGFloat = 0

    var body: some View {
        ScrollView {
            VStack {
                Text("Offset: \(scrollViewOffset)")
                
                ForEach(0 ..< 100) { _ in
                    Text("Hello World!")
                }
            }
            .frame(maxWidth: .infinity)
        }
        .scrollViewOffset($scrollViewOffset)
    }
}