> In other words, contrary to your earlier dismissal, you did NOT
run _decimal on AIX with MAX_PREC but just ran the 3.7.6 tests
that do not include any tests with MAX_PREC.

I did, and it raises MemoryError:

❯ uname -a
AIX 1 7 powerpc 00CCAD974C00 AIX

❯ python3.7 --version
Python 3.7.6

❯ python3.7 Lib/test/test_decimal.py

...

======================================================================
ERROR: test_maxcontext_exact_arith (__main__.CWhitebox)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_decimal.py", line 5506, in test_maxcontext_exact_arith
    self.assertEqual(Decimal(4).sqrt(), 2)
MemoryError