I am not sure if we support it already for sag script, but the SVG WG wants to support the attributes 'async' and 'defer' for <svg:script> as well. If this bug is invalid feel free to close it.
If I am not mistaken, we have WPT tests here: https://wpt.fyi/results/svg/interact/scripted/defer-01.svg?label=master&label=experimental&aligned & https://wpt.fyi/results/svg/interact/scripted/async-01.svg?label=master&label=experimental&aligned @Anne - should it be in "SVG" componenet?