Its also really not that hard to make cmd click + back button to work properly with client side nav.
If on the other hand, if you encounter code like the abomination below (which breaks cmd+click, peek and forward/back), you should not (imo) conflate it with actual client-side navigation. I suspect that's what some comments here are referring to.
<a href="#" onclick="loadContentIntoPane('contact')">Contact Us</a>
1. https://developer.mozilla.org/en-US/docs/Web/API/History_API