However, sentinels *are* mentioned in the multiprocessing doc, below multiprocessing.Process:

"
sentinel

    A numeric handle of a system object which will become “ready” when the process ends.

    On Windows, this is an OS handle usable with the WaitForSingleObject and WaitForMultipleObjects family of API calls. On Unix, this is a file descriptor usable with primitives from the select module.

    You can use this value if you want to wait on several events at once. Otherwise calling join() is simpler.

    New in version 3.3.
"

From a cursory glance on the implementation in Lib/multiprocessing/connection.py, this is the same sentinel.