Any single page application, such as YouTube, Gmail, or discord.

It lets persistent content (videos) or connections (chat) persist while emulating a pagenated browsing experience.

When it's done right you don't notice it at all.

Youtube doesn't implement a back function. A real back function would take you back to the same page you came from. If you click a video from the Youtube home page, then click the back button, Youtube will regenerate a different home page with different recommendations, losing the potentially interesting set of recommendations you saw before. You are forced to open every link in a new tab if you want true back functionality.

(rant warning)

Well, if I wanted to return to the parent screen in a single page application, I'd click on the back button in the app itself. No need to prevent me from back tracking in the exact order of my browsing should I need it.

I especially hate YouTube's implementation, I can never know the true state on my older PC during whatever it's trying to accomplish, often playing audio from a previous video when I backspace out. I resort to opening every link in a new tab.