TextField has a .autocapitalization() method.
You can use like this without custom binding:
TextField("URL", text: $url)
.keyboardType(.URL)
.autocapitalization(.none)
For iOS 15 SwiftUI have a new .textInputAutocapitalization() method:
.textInputAutocapitalization(.never)
This means that any text input by the user will be .lowercased()