Specifically, in Javascript you can superficially push a navigated state into the location stack so that when the user presses the back button, it is now the inserted page instead of the “actual” one. Most FE devs consider this a dark pattern and refuse to implement it.
That said, it’s basically just the front end’s version of a redirect. Unfortunately it’s been used a lot especially by malware sites to basically jail people into their site.
I tried to when I worked at The Huffington Post a long time ago and was ridiculed. Some people don't care so they can point to increased metrics, ethics be damned.
No, it wouldn't be that. That can only prevent unload if it throws a native modal up (cousin of alert, confirm, prompt, etc).
I think what they must have done is push some URL to the history state, using the History API, to fuck with the back and forward buttons, enabling them to intercept when you navigate back, by having added an additional page in there, that you never visited, but that they can use to serve you this interstitial.
That said, it’s basically just the front end’s version of a redirect. Unfortunately it’s been used a lot especially by malware sites to basically jail people into their site.