Reverting the code will cause another class of problems, like the reason I fixed it. Programs written such as the example that Pablo gave (and what I've seen) will quietly leak child processes, file descriptors (for the pipes) and memory to a variety degree might not be detected, or in the end detected in a big error or crash.

Also in some ResourceWarnings (if not all), the resources are closed in the end (like in sockets), here without this code patch you cannot implicitly reclaim the resources (because there is a Thread involved here), which I think is a high bar for the user to think about.

You can also enable multiprocessing's debug logging to see how the code behaves with and without the fix:
https://stackoverflow.com/a/1353037

I also agree with Pablo that there is code in the stdlib that holdes reference between child and parent. There is also code that has circular reference (for example Python 2's OrderedDict) and that is ok as well (not that this is the situation here).