According to a recent thread on python-dev, Python APIs with the same
name as native APIs can be trusted to just pass through to the platform
API call
(<http://mail.python.org/pipermail/python-dev/2009-January/084899.html>). 
This isn't the case for the environment APIs in the os module.  For
example, os.getenv doesn't always return the actual value of a key in
the environment.  Instead, it looks in the os.environ pseudo-dict to get
the answer.  This can be wrong because os.environ is a *copy* of the
environment which any non-Python API will use, and as a copy it can get
out of sync with the real environment.  One way it can get out of sync
is via os.environ.pop (at least on POSIX - os.environ has different
implementations on different platforms, I find the way an implementation
is selected to be rather confusing and I have not tracked down the
precise behavior for all platforms):

>>> import os
>>> os.environ.pop('PATH')
'/home/exarkun/.local/sbin:/home/exarkun/.local/bin:/home/exarkun/Projects/combinator_paths/bincache:/home/exarkun/.local/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11:/usr/games'
>>> os.getenv('PATH')
>>> os.system("echo $PATH")
/home/exarkun/.local/sbin:/home/exarkun/.local/bin:/home/exarkun/Projects/combinator_paths/bincache:/home/exarkun/.local/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11:/usr/games
0
>>> 

Another way is via calls to the platform environment manipulation APIs
(setenv, unsetenv or putenv).  These might be invoked by a third-party
library which is exposed to Python or via ctypes.

`environ´ itself is a C API, though it's a char**, whereas os.environ is
a Python dict.  It's probably convenient for os.environ to remain as a
dict, since that's so much simpler to manipulate and inspect than the C
data structure used.  However, it'd be good if were always in sync with
the real process environment.  A good way to do this would probably be
to stop maintaining a copy of the environment and always pass through to
one of the platform APIs to satisfy a method call onto it.

Is the OP interested in taking this forward?

What are the chances a future Python 2.x release will include any fix developed for this issue?

Good as 2.7 is in support until 2020.

I suspect the $PATH example is a bad example. On my Linux setup, my initial PATH value is a customized path. After popping it and calling system(), I see a simpler default path, which is probably re-initialized somewhere else.

I suspect there is no environ.pop() bug. Everthing works according the documentation if you use another environment variable:

$ TEST_VARIABLE=value python
Python 3.4.0 (default, Mar 17 2014, 23:20:09) 
[GCC 4.8.2 20140206 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.system('echo "$TEST_VARIABLE"')
value
0
>>> os.environ.pop("TEST_VARIABLE")
'value'
>>> os.system('echo "$TEST_VARIABLE"')

0

The other point in this report, about synchronizing “os.environ” with the C APIs, is a duplicate of Issue 1159.

Agreed. environ.pop() was fixed a long time ago with issue1287.
It seems that all mutable methods of the environ pseudo-dict are now correctly reflected to the platform environ.

The other direction (updates from C code should be reflected in os.environ) is tracked by issue1159.