AnimatableModifier已弃用。这不需要AnimatableMask。您可以只使用带有value:参数的animation修改器来设置蒙版的动画

// Shimmer.body
public func body(content: Content) -> some View {
    content
        .mask(
            GradientMask(phase: phase).scaleEffect(3)
        )
        .animation(animation, value: phase)
        .onAppear { phase = 0.8 }
}

也就是说，我不知道为什么scaleEffect(3)或者梯度的逻辑是如何停止工作的。scaleEffect(3)只是让整个渐变变得非常大，你根本看不清动画。如果删除scaleEffect，动画看起来也不那么好。
也许梯度停止工作的方式在某个时候改变了。
我假设this是你得到代码的地方。看看这个repo的历史，作者从动画停止变成了动画渐变的开始和结束点，这对我来说更有意义，实际上看起来很不错。
代码如下：

public struct Shimmer: ViewModifier {
    private let animation: Animation
    private let gradient: Gradient
    private let min, max: CGFloat
    @State private var isInitialState = true
    @Environment(\.layoutDirection) private var layoutDirection

    /// Initializes his modifier with a custom animation,
    /// - Parameters:
    ///   - animation: A custom animation. Defaults to ``Shimmer/defaultAnimation``.
    ///   - gradient: A custom gradient. Defaults to ``Shimmer/defaultGradient``.
    ///   - bandSize: The size of the animated mask's "band". Defaults to 0.3 unit points, which corresponds to
    /// 30% of the extent of the gradient.
    public init(
        animation: Animation = Self.defaultAnimation,
        gradient: Gradient = Self.defaultGradient,
        bandSize: CGFloat = 0.3
    ) {
        self.animation = animation
        self.gradient = gradient
        // Calculate unit point dimensions beyond the gradient's edges by the band size
        self.min = 0 - bandSize
        self.max = 1 + bandSize
    }

    /// The default animation effect.
    public static let defaultAnimation = Animation.linear(duration: 1.5).delay(0.25).repeatForever(autoreverses: false)

    // A default gradient for the animated mask.
    public static let defaultGradient = Gradient(colors: [
        .black.opacity(0.3), // translucent
        .black, // opaque
        .black.opacity(0.3) // translucent
    ])

    /*
     Calculating the gradient's animated start and end unit points:
     min,min
        \
         ┌───────┐         ┌───────┐
         │0,0    │ Animate │       │  "forward" gradient
     LTR │       │ ───────►│    1,1│  / // /
         └───────┘         └───────┘
                                    \
                                  max,max
                max,min
                  /
         ┌───────┐         ┌───────┐
         │    1,0│ Animate │       │  "backward" gradient
     RTL │       │ ───────►│0,1    │  \ \\ \
         └───────┘         └───────┘
                          /
                       min,max
     */

    /// The start unit point of our gradient, adjusting for layout direction.
    var startPoint: UnitPoint {
        if layoutDirection == .rightToLeft {
            return isInitialState ? UnitPoint(x: max, y: min) : UnitPoint(x: 0, y: 1)
        } else {
            return isInitialState ? UnitPoint(x: min, y: min) : UnitPoint(x: 1, y: 1)
        }
    }

    /// The end unit point of our gradient, adjusting for layout direction.
    var endPoint: UnitPoint {
        if layoutDirection == .rightToLeft {
            return isInitialState ? UnitPoint(x: 1, y: 0) : UnitPoint(x: min, y: max)
        } else {
            return isInitialState ? UnitPoint(x: 0, y: 0) : UnitPoint(x: max, y: max)
        }
    }

    public func body(content: Content) -> some View {
        content
            .mask(LinearGradient(gradient: gradient, startPoint: startPoint, endPoint: endPoint))
            .animation(animation, value: isInitialState)
            .onAppear {
                isInitialState = false
            }
    }
}

public extension View {
    /// Adds an animated shimmering effect to any view, typically to show that an operation is in progress.
    /// - Parameters:
    ///   - active: Convenience parameter to conditionally enable the effect. Defaults to `true`.
    ///   - animation: A custom animation. Defaults to ``Shimmer/defaultAnimation``.
    ///   - gradient: A custom gradient. Defaults to ``Shimmer/defaultGradient``.
    ///   - bandSize: The size of the animated mask's "band". Defaults to 0.3 unit points, which corresponds to
    /// 20% of the extent of the gradient.
    @ViewBuilder func shimmering(
        active: Bool = true,
        animation: Animation = Shimmer.defaultAnimation,
        gradient: Gradient = Shimmer.defaultGradient,
        bandSize: CGFloat = 0.3
    ) -> some View {
        if active {
            modifier(Shimmer(animation: animation, gradient: gradient, bandSize: bandSize))
        } else {
            self
        }
    }

    /// Adds an animated shimmering effect to any view, typically to show that an operation is in progress.
    /// - Parameters:
    ///   - active: Convenience parameter to conditionally enable the effect. Defaults to `true`.
    ///   - duration: The duration of a shimmer cycle in seconds.
    ///   - bounce: Whether to bounce (reverse) the animation back and forth. Defaults to `false`.
    ///   - delay:A delay in seconds. Defaults to `0.25`.
    @available(*, deprecated, message: "Use shimmering(active:animation:gradient:bandSize:) instead.")
    @ViewBuilder func shimmering(
        active: Bool = true, duration: Double, bounce: Bool = false, delay: Double = 0.25
    ) -> some View {
        shimmering(
            active: active,
            animation: .linear(duration: duration).delay(delay).repeatForever(autoreverses: bounce)
        )
    }
}