bpo-38323: Change MultiLoopChildWatcher to install handlers for all the event loops by shreyanavigyan · Pull Request #26574 · python/cpython

IMO, it's better to deprecate the class in 3.10 and remove it in 3.12, and for 3.9 do nothing. But, like Victor, I'm not an asyncio user, nor do I have any knowledge of that module, so my opinion does not really matter here.