Fisheye was only an example. It is about adding any camera lens you want, that’s why Unity exposed it. Doing it in post processing is out of question because you need to oversample, which is just nonsense if you can modify the projection matrix directly. And even with oversampling you will still notice that the stretched center pixel for example were simply magnified.
Here is postprocessing in case someone wants to do it. You really just take the Screen texture (the render results) and stretch it like in Photoshop&Co… By nature of the process, you get really ugly artifacts as soon as you push it.