> > The child watchers API has to go. It's confusing, painful to use, it's not compatible with third-party event loops. It increases the API surface without providing us with enough benefits.

> +1

Also, adding to this, the child watchers are one of the least used components of asyncio's public API. So, I think the deprecation and removal cost will be fairly minimal. See the GitHub code usage (includes exact copies of Lib/asyncio/unix_events.py, so there's some redundancy):

MultiLoopChildWatcher: https://github.com/search?l=Python&q=MultiLoopChildWatcher&type=Code (20 results, just added in 3.8)
ThreadedChildWatcher: https://github.com/search?l=Python&q=ThreadedChildWatcher&type=Code (77 results, default unix child watcher, rarely used explicitly)
FastChildWatcher: https://github.com/search?l=Python&q=FastChildWatcher&type=Code (4,426 results)
SafeChildWatcher: https://github.com/search?l=Python&q=SafeChildWatcher&type=Code (7,007 results)
All of asyncio usage: https://github.com/search?l=Python&q=asyncio&type=Code (599,131 results)