I've reviewed the main branch and the GIL isn't released: https://github.com/python/cpython/blob/main/Modules/posixmodule.c#L7060

The forum thread: https://discuss.python.org/t/does-the-os-sched-yield-release-the-gil/18301

Expected: The GIL is released before sched_yield() and acquired after that.