The parent likely means background killed in the sense that iOS fairly aggressively removes loaded web pages from memory and replaces them with a screenshot and

The parent likely means background killed in the sense that iOS fairly aggressively removes loaded web pages from memory and replaces them with a screenshot and a url. Most of your 500 tabs are likely screenshots and urls. When you select them they fetch the page from the server fresh. In most cases this is fine and frees up memory for other apps/tabs, but with web apps with local state, said state gets clobbered in a way that never happens on a desktop browser. Because it is fully automatic you can never really tell or trust that local application state will still exist if you tab away and back.