It depends on Python's socket implementation.
If CPython, PyPy and other implmentaions are safe, we can write "This is fork-safe."

I'm interested in this as well, having some issues getting async handler sending to a remote fluentd host with a basic gunicorn + Django deployment

I don't think It's definitely not fork-safe. For my example above, I needed make sure workers weren't preloaded to get the asynchandler.FluentHandler working. This is because gunicorns forks workers and the logging initialization happens pre-fork, in which a daemon is created by the async sender.

benoitc/gunicorn#1045 (comment)

Generally-speaking NO threaded application is fork-safe. Forks and threads are not compatible as POSIX mandates killing all threads upon forking. Sync sender should be safe.

If you want the fD survive you will need to make sure the worker inherit from it at the system level. This can be done similarly to what does the logging handler.