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--> s = []
--> s += 1
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
For comparison, when NotImplemented is appropriately returned the message looks like this:
--> s -= 1
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -=: 'list' and 'int'
Which is certainly more helpful than "<blank> object is not iterable"

Please don't replicate existing issues.
Also, this behavior for lists is long-standing, tested, and designed by Guido. More than a little code relies on this behavior.

My understanding was that if a different patch will be needed, it is not the same issue. Is that not correct?
Here's the test:
 def test_iadd(self):
 ...
 self.assertRaises(TypeError, u.__iadd__, None)
It would still pass, as returning NotImplemented will still result in a TypeError, just with a friendlier message.
If any code is depending on this, it is depending on the error message text, which is explicitly not guaranteed