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I would expect this to be a difference between the original pure Python implementation and the current C accelerated implementation.
The constraint is in alignment with the General Decimal Arithmetic Specification that the decimal module aims to implement, which requires that Emin and Emax define a balanced range about zero. (That is, Emin = -Emax +/- 1)
The fact the pure Python implementation didn't enforce that constraint was an implementation detail - other implementations (including the cdecimal module, which was incorporated into the standard library to provide CPython's C accelerated version in Python 3.3) are free to enforce it.