您可以使用ScrollViewReader容器视图，它提供了一种机制来读取滚动视图的内容偏移量，并使用scrollTo方法滚动到其中的特定材料。以下是更新的代码：

struct Testing: View {
    let years = ["2016", "2017", "2018", "2019", "2020", "2021", "2022", "2023", "2024", "2025", "2026"]
    @State var selectedYear = "2023"
    @Namespace private var animation
    
    var body: some View {
        
        ScrollView(.horizontal, showsIndicators: false) {
            ScrollViewReader { scrollView in // --> Here
                HStack(spacing: 20) {
                    
                    ForEach(years.indices, id: \.self) { index in
                        
                        Button {
                            withAnimation(.linear(duration: 0.5)) {
                                selectedYear = years[index]
                            }
                        } label: {
                            VStack {
                                Text("\(years[index])")
                                    .foregroundColor(.white)
                                    .fontWeight(.semibold)
                                    .padding(.horizontal, 15)
                                
                                if selectedYear == years[index] {
                                    Capsule()
                                        .fill(.white)
                                        .frame(height: 5)
                                        .padding(.horizontal, -5)
                                }
                            }
                            .id(years[index]) // ---> Here
                        }
                    }
                }
                .padding().background {
                    Rectangle()
                        .fill(Color.purple)
                }
                .onAppear {
                    scrollView.scrollTo(selectedYear, anchor: .center) // ---> Here
                }
            }
        }
    }
}