As of February 4th I intend to turn support for the SVG discard element on by default. It has been developed behind the svg.discard.enabled preference and is enabled in nightlies.
Status in other browsers is Chrome used to ship this and removed support due to lack of other implementations and flaky tests. I imagine they might add it back again once we ship and now that the tests are not flaky. Safari has not implemented this element but presumably could adapt the Chrome implementation.
Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1945330.
Standard: https://svgwg.org/specs/animations/#DiscardElement
This feature was previously discussed in this "Intent to prototype" thread: https://groups.google.com/a/mozilla.org/g/dev-platform/c/U4nufgUTdAk