https://docs.python.org/3/c-api/objbuffer.html
Old buffer protocol has been deprecated since Python 3.0.
It was useful to make transition from Python 2 easy.
But it's time to remove.

New changeset 6f8a6ee59cb7f99f68df8ee9c3e8c8cf19af3eed by Inada Naoki in branch 'master':
bpo-41103: Remove old buffer protocol support (#21117)
https://github.com/python/cpython/commit/6f8a6ee59cb7f99f68df8ee9c3e8c8cf19af3eed

I've just seen a fix of PyQt4 that basically copy pastes (some of) the removed code to PyQt4:

https://src.fedoraproject.org/rpms/PyQt4/pull-request/2#request_diff

What is the benefit of removing this? Is copy pasting the compatibility layer to (possibly many) different projects worth the "cleanup"?

In Fedora, at least 7 packages are broken so far:

PyQt4: https://bugzilla.redhat.com/show_bug.cgi?id=1895298
libsolv: https://bugzilla.redhat.com/show_bug.cgi?id=1896411
m2crypto: https://bugzilla.redhat.com/show_bug.cgi?id=1897148
apsw: https://bugzilla.redhat.com/show_bug.cgi?id=1897500
djvulibre: https://bugzilla.redhat.com/show_bug.cgi?id=1897558
wsaccel: https://bugzilla.redhat.com/show_bug.cgi?id=1899550
webassets: https://bugzilla.redhat.com/show_bug.cgi?id=1899555

PyObject_AsCharBuffer() is dangerous: it returns a dangling pointer by design. PyObject_GetBuffer() design is safer: the API ensures that the buffer remains valid until PyBuffer_Release() is called.

PyQt5 was updated to use the safe PyObject_GetBuffer()/PyBuffer_Release(). PyQt4 is no longer updated, that's why I proposed a downstream fix which copy/paste the old code. Changing PyQt4 to use the safe API was not worth it, since it's complex to redesign the impacted function qpycore_encode().

Thank you for reporting. I removed these APIs to get such feedback as early as possible.

We are free to revive these APIs if it breaks too much and some projects can not be fixed before Python 3.10 release.

Some project maintainers ignore deprecations and wait compile errors to fix soemthing. (e.g. https://github.com/rogerbinns/apsw/issues/288#issuecomment-644442322)
That's why we need to break some projects during alpha phase.


> What is the benefit of removing this? Is copy pasting the compatibility layer to (possibly many) different projects worth the "cleanup"?

Oh, no need to copy-paste compatibility layer for all projects. They can migrate to new APIs.