Animate a view to slide up and hide on tap in SwiftUI

Removing view from hierarchy is always animated by container, so to fix your modifier it is needed to apply .animation on some helper container (note: Group is not actually a real container).

Here is corrected variant

struct BannerModifier: ViewModifier {
    @Binding var model: BannerData?
    
    func body(content: Content) -> some View {
        content.overlay(
            VStack {               // << holder container !!
                if model != nil {
                    VStack {
                        HStack(alignment: .firstTextBaseline) {
                            Image(systemName: "exclamationmark.triangle.fill")
                            VStack(alignment: .leading) {
                                Text(model?.title ?? "")
                                    .font(.headline)
                                if let message = model?.message {
                                    Text(message)
                                        .font(.footnote)
                                }
                            }
                        }
                        .padding()
                        .frame(minWidth: 0, maxWidth: .infinity)
                        .foregroundColor(.white)
                        .background(Color.red)
                        .cornerRadius(10)
                        .shadow(radius: 10)
                        Spacer()
                    }
                    .padding()
                    .transition(AnyTransition.move(edge: .top).combined(with: .opacity))
                    .onTapGesture {
                        withAnimation {
                            model = nil
                        }
                    }
                    .gesture(
                        DragGesture()
                            .onChanged { _ in
                                withAnimation {
                                    model = nil
                                }
                            }
                    )
                }
            }
            .animation(.easeInOut)         // << here !!
        )
    }
}

Tested with Xcode 12.1 / iOS 14.1 and test view:

struct TestBannerModifier: View {
    @State var model: BannerData?
    var body: some View {
        VStack {
            Button("Test") { model = BannerData(title: "Error", message: "Fix It!")}
            Button("Reset") { model = nil }
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .modifier(BannerModifier(model: $model))
    }
}

This is a slightly improoved version of the banner posted by Asperi Hope it helps someone.

import SwiftUI

public class BannerData {
    public enum BannerType {
        case warning, error, success

        var textColor: Color {
            switch self {
            case .warning:
                return .black
            case .error:
                return .white
            case .success:
                return .white
            }
        }

        var backgroundColor: Color {
            switch self {
            case .warning:
                return .yellow
            case .error:
                return .red
            case .success:
                return .green
            }
        }

        var icon: String {
            switch self {
            case .warning:
                return "exclamationmark.triangle.fill"
            case .error:
                return "exclamationmark.circle.fill"
            case .success:
                return "checkmark.circle.fill"
            }
        }
    }

    var type: BannerType = .success
    let title: String
    let message: String?

    public init(title: String, message: String? = nil, type: BannerType) {
        self.title = title
        self.message = message
        self.type = type
    }
}

public struct BannerModifier: ViewModifier {
    @Binding var model: BannerData?

    public init(model: Binding<BannerData?>) {
        _model = model
    }

    public func body(content: Content) -> some View {
        content.overlay(
            VStack {
                if model != nil {
                    VStack {
                        HStack(alignment: .firstTextBaseline) {
                            Image(systemName: model?.type.icon ?? BannerData.BannerType.success.icon)
                            VStack(alignment: .leading) {
                                Text(model?.title ?? "")
                                    .font(.headline)
                                if let message = model?.message {
                                    Text(message)
                                        .font(.footnote)
                                }
                            }
                            Spacer()
                        }
                        .padding()
                        .frame(minWidth: 0, maxWidth: .infinity)
                        .foregroundColor(.white)
                        .background(model?.type.backgroundColor ?? .clear)
                        .cornerRadius(10)
                        .shadow(radius: 10)
                        Spacer()
                    }
                    .padding()
                    .transition(AnyTransition.move(edge: .top).combined(with: .opacity))
                    .onTapGesture {
                        withAnimation {
                            model = nil
                        }
                    }
                    .gesture(
                        DragGesture()
                            .onChanged { _ in
                                withAnimation {
                                    model = nil
                                }
                            }
                    )
                }
            }
            .animation(.spring())
        )
    }
}