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The function was added by:
commit ae3087c6382011c47db82fea4d05f8bbf514265d
Author: Mark Shannon <mark@hotpy.org>
Date: Sun Oct 22 22:41:51 2017 +0100
 Move exc state to generator. Fixes bpo-25612 (#1773)
 
 Move exception state information from frame objects to coroutine (generator/thread) object where it belongs.
Python 3.9 is now built with -fvisibility=hidden, bpo-11410:
commit 0b60f64e4343913b4931dc27379d9808e5b78fe1
Author: Vinay Sajip <vinay_sajip@yahoo.co.uk>
Date: Tue Oct 15 08:26:12 2019 +0100
 bpo-11410: Standardize and use symbol visibility attributes across POSIX and Windows. (GH-16347)
It means that functions which are not explicitly exported are no longer exported.
So yeah, _PyErr_GetTopmostException() symbol is not properly exported.
But the function remains private. IMHO if someone wants to add a *public* function, it should return a *strong* reference to the 3 variables (exc type, exc value, exc tb).
See also bpo-39947: "[C API] Make the PyThreadState structure opaque (move it to the internal C API)". I added multiple public getters for PyThreadState structure in Python 3.9.