Serhiy was right, this is a security issue.

The patch should not have landed in 3.8. At a bare minimum the patch should have been postponed until documentation was updated. Since 3.8 the ipaddresss does not behave as documented. A similar security issue in NPM was published two days ago, CVE-2021-28918.

I proposed to not only revert the change, but also tighten the check for leading zeros so it behaves like glibc's inet_pton(). It refuses any IPv4 string with a leading zero.

>>> socket.inet_pton(socket.AF_INET, "01.1.1.1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: illegal IP address string passed to inet_pton
>>> socket.inet_pton(socket.AF_INET, "1.1.1.01")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: illegal IP address string passed to inet_pton