The <discard> element is part of the SVG Animations (draft) specification. It provides a way to declaratively remove elements at a specific point in time and be a mechanism for streaming animations.
The <discard> element is only implemented in Blink, and is thus not possible to use interoperably. For most use cases it can be replaced with a combination of animation of the 'display' property and a removal (JavaScript) callback/event handler.