List of the 58 C extensions using the legacy API:

_asynciomodule.c
_bisectmodule.c
_blake2/blake2module.c
cjkcodecs/multibytecodec.c
cmathmodule.c
_csv.c
_ctypes/_ctypes.c
_cursesmodule.c
_curses_panel.c
_datetimemodule.c
_decimal/_decimal.c
_elementtree.c
faulthandler.c
gcmodule.c
grpmodule.c
_hashopenssl.c
_io/_iomodule.c
_lsprof.c
md5module.c
_multiprocessing/multiprocessing.c
_multiprocessing/posixshmem.c
_opcode.c
ossaudiodev.c
overlapped.c
_pickle.c
_posixsubprocess.c
pwdmodule.c
pyexpat.c
_queuemodule.c
_randommodule.c
readline.c
_scproxy.c
selectmodule.c
sha1module.c
sha256module.c
_sha3/sha3module.c
sha512module.c
signalmodule.c
socketmodule.c
spwdmodule.c
_sqlite/module.c
_sre.c
_ssl.c
_struct.c
symtablemodule.c
termios.c
_testbuffer.c
_testcapimodule.c
_testimportmultiple.c
_testinternalcapi.c
_threadmodule.c
_tkinter.c
_tracemalloc.c
unicodedata.c
_winapi.c
_xxsubinterpretersmodule.c
_xxtestfuzz/_xxtestfuzz.c
zlibmodule.c