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Python has an internal function to get the frame of the PyThreadState:
/* hook for PyEval_GetFrame(), requested for Psyco */
#define _PyThreadState_GetFrame _PyRuntime.gilstate.getframe
It is used by the public function PyEval_GetFrame() for example.
The indirection was added in 2002 by:
commit 019a78e76d3542d4d56a08015e6980f8c8aeaba1
Author: Michael W. Hudson <mwh@python.net>
Date: Fri Nov 8 12:53:11 2002 +0000
 Assorted patches from Armin Rigo:
 
 [ 617309 ] getframe hook (Psyco #1)
 [ 617311 ] Tiny profiling info (Psyco #2)
 [ 617312 ] debugger-controlled jumps (Psyco #3)
 
 These are forward ports from 2.2.2.
... but psyco is outdated for a very long time (superseded by PyPy which is no longer based on CPython). Is it time to drop _PyThreadState_GetFrame() (which became _PyRuntime.gilstate.getframe in the meanwhile)?
Or if we keep it, we should use it rather accessing directly PyThreadState.frame (read or write).
See also PEP 523 "Adding a frame evaluation API to CPython" and a recent discussion on this PEP: bpo-38500.

I added PyThreadState_GetFrame() function in bpo-39947: commit fd1e1a18fa3befe5b6eeac32e0561e15c7e5164b.

It looks safe to remove this feature.
I failed to find any recent user of _PyThreadState_GetFrame.
Moreover, _PyRuntime.getframe and _PyThreadState_GetFrame have been moved to the internal C API in Python 3.7. This C API was not accessible by third-party projects in Python 3.7. The internal C API can only be used by third-party projects since Python 3.8 and it's tricky to use it... on purpose. I don't recall any complain about _PyThreadState_GetFrame becoming inaccessible.
--
I searched for "_PyThreadState_GetFrame" in C code on GitHub. I only found copies of Python/pystate.c files in the first 10 pages of search results, with one exception. I found one file:
https://github.com/Mistobaan/pdb-clone/blob/517f6d19902b64395b4c7218cbbbecfa5a1de607/lib/pdb_clone/_pdbhandler-py27.c
It's an old (latest commit on Mar 31, 2015) debugger project written by Xavier de Gaye.
It seems like the following flavor is more recent (latest commit on Apr 20, 2019):
https://github.com/corpusops/pdbclone/
Note: the project contains 4 .c files, but only _pdbhandler-py27.c uses _PyThreadState_GetFrame: _pdbhandler-py3.c which is for Python 3 doesn't use _PyThreadState_GetFrame. But Python 2.7 reached its end of life, it's no longer supported.
Copy of the code:
 /* Disable the Python 2 restricted mode in the subinterpreter (see
 * PyEval_GetRestricted()) that prevents linecache to open the source
 * files and prevents attribute access. */
 saved_globals = mainstate->frame->f_globals;
 saved_locals = mainstate->frame->f_locals;
 saved_tstate_getframe = _PyThreadState_GetFrame;
 mainstate->frame->f_globals = globals;
 mainstate->frame->f_locals = locals;
 _PyThreadState_GetFrame = threadstate_getframe;
 pdbhandler_tstate = mainstate;
So _PyThreadState_GetFrame was used to "disable the Python 2 restricted mode", but this mode has been removed from Python 3.

New changeset 6723e933c4d90a408cf3818362a0e4de6d84c932 by Victor Stinner in branch 'master':
bpo-39946: Remove _PyThreadState_GetFrame (GH-19094)
https://github.com/python/cpython/commit/6723e933c4d90a408cf3818362a0e4de6d84c932

I removed the function.
FYI I asked Armin Rigo, the author of _PyThreadState_GetFrame, if PyPy uses it: no, it doesn't. Moreover, psyco project is outdated, only supports Python 2.6 and older, and is superseded by PyPy.

Thanks!
We use this function internally in some VM traceback grabbing code but the best solution looks to just be for us to adopt the patch to 3.9 on our interpreter until we're running on 3.9.

> We use this function internally in some VM traceback grabbing code
Would you mind to elaborate your use case?