Short Version:
root@x065:[/data/prj/python/python3-3.8]./python -m test test_subprocess
Run tests sequentially
0:00:00 [1/1] test_subprocess
test_subprocess passed in 2 min 18 sec

== Tests result: SUCCESS ==

1 test OK.

Total duration: 2 min 18 sec
Tests result: SUCCESS

...
----------------------------------------------------------------------
Ran 285 tests in 136.443s

OK (skipped=33)
test_subprocess passed in 2 min 16 sec

== Tests result: SUCCESS ==

1 test OK.

Total duration: 2 min 16 sec
Tests result: SUCCESS

Posting a PR - tested manually on AIX 5.3, AIX 6.1 and AIX 7.1

imho: being able to remove the special condition for AIX is an improvement.

diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 73b57b21db..4719773b67 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -2228,15 +2228,9 @@ class POSIXProcessTestCase(BaseTestCase):
             env = os.environ.copy()
             env[key] = value
             # Use C locale to get ASCII for the locale encoding to force
-            # surrogate-escaping of \xFF in the child process; otherwise it can
-            # be decoded as-is if the default locale is latin-1.
+            # surrogate-escaping of \xFF in the child process
             env['LC_ALL'] = 'C'
-            if sys.platform.startswith("aix"):
-                # On AIX, the C locale uses the Latin1 encoding
-                decoded_value = encoded_value.decode("latin1", "surrogateescape")
-            else:
-                # On other UNIXes, the C locale uses the ASCII encoding
-                decoded_value = value
+            decoded_value = value
             stdout = subprocess.check_output(
                 [sys.executable, "-c", script],
                 env=env)