Well plugging an iPhone 11 into the wall with the included cable would be 5W and the phone itself only supports up to 18W. So yeah plugging it into the wall will be super slow too.
So yes some people will buy a USB C to lightning cable for their older iPhone and a better wall adapter but most people will just use the included charger.
Why would you do that? It's going to be super-slow - I don't believe it supports Fast Charging as the port can't handle the minimum 20 W.