How can I unwrap an optional value inside a binding in Swift?

You can use this initialiser, which seems to handle this exact case – converting Binding<T?> to Binding<T>?:

var body: some View {
    AvatarView(userData: Binding($userById[activeUserId])!)
}

I have used ! to force unwrap, just like in your attempts, but you could unwrap the nil however you want. The expression Binding($userById[activeUserId]) is of type Binding<UserData>?.