• bitarray: bitarray/_bitarray.c uses Py_SET_SIZE() (pythoncapi_compat.h copy)
• datatable (commit)
• guppy3 (commit)
• immutables: immutables/_map.c uses Py_SET_SIZE() (pythoncapi_compat.h copy)
• Mercurial (hg) uses Py_SET_TYPE() (commit, pythoncapi_compat.h copy)
• mypy (mypyc, commit)
• numpy: pythoncapi-compat Git submodule
• pybluez (commit)
• python-snappy (commit)
• python-zstandard uses Py_SET_TYPE() and Py_SET_SIZE() (commit): Mercurial extension.
• python-zstd (commit)
• hollerith src/writer.c uses PyObject_CallOneArg() and other Python 3.9 apis (pythoncapi_compat.h copy)
• PyTorch (pythoncapi_compat.h copy)
• PyGObject (commit, pythoncapi-compat Meson subproject)

Projects not using pythoncapi_compat.h:

• numpy has its own compatibility layer, npy_pycompat.h and npy_3kcompat.h header files. It supports more C compilers than pythoncapi_compat.h: it supports __STRICT_ANSI__ (ISO C90) for example. Rejected PR 18713: MAINT: Use pythoncapi_compat.h in npy_3kcompat.h (when it was rejected, numpy still had code for compatibility with Python 2.7).
• Cython doesn't use pythoncapi_compat.h: see Cython issue #3934. For example, ModuleSetupCode.c provides functions like __Pyx_SET_REFCNT().

Project with a strict contributor agreement:

• zodbpickle