If you can compute the points that define the corners of the shape then an easy way to get a rounded shape is to supply the points to RoundedShape. This is a generic wrapper for a set of points that rounds all the corners automatically. It can be found in the answer to Adding rounded corners to custom SwiftUI shape? (it was my answer).

The basic shape here is a rectangle with a lower top-left corner. Computing the four points for the corners of the shape is therefore quite easy, you just need a way to determine how much shorter the leading side will be. One way is to make the reduction in height a fixed fraction of the width:

struct SlopedShape: Shape {
    let slopeFraction: CGFloat
    var cornerRadius: CGFloat = 16

    func path(in rect: CGRect) -> Path {

        // See /a/78497329/20386264
        RoundedShape(
            points: [
                CGPoint(x: rect.minX, y: rect.maxY),
                CGPoint(x: rect.minX, y: rect.minY + rect.width * slopeFraction),
                CGPoint(x: rect.maxX, y: rect.minY),
                CGPoint(x: rect.maxX, y: rect.maxY)
            ],
            cornerRadius: cornerRadius
        )
        .path(in: rect)
    }
}

If you have multiple shapes that are next to each other and you want the slope to look continuous, apply top padding to the leading shapes. The height of the padding should correspond to the slope height of the remaining width (including spacing).

Example use:

struct ContentView: View {
    let slopeFraction: CGFloat = 0.4
    let spacing: CGFloat = 8

    var body: some View {
        HStack(spacing: spacing) {
            SlopedShape(slopeFraction: slopeFraction)
                .padding(.top, (50 + spacing) * slopeFraction)
                .frame(width: 100)
            SlopedShape(slopeFraction: slopeFraction)
                .frame(width: 50)
        }
        .foregroundStyle(.gray)
        .frame(height: 120)
    }
}