Currently Chromium-based browsers are the only ones that fire blur events when an element is removed from the DOM. This has recently been clarified in the spec. As part of an effort to reduce synchronous side-effects of DOM insertion & removal, this deprecation will cautiously test the impact and feasibility of aligning with the spec/other browsers on this.
This is a current interop issue, as blink is the only engine that does this. It's an important one, as this is one of the only remaining places where a DOM change can trigger synchronous scripts, which causes bugs and unpredictable behavior.