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There is indeed a regression in decimal.localcontext with the C accelerator: omitting the argument entirely still works, but passing None explicitly now fails. I created #15783 for that.
On the topic of this thread, I'm also -1 on conversion methods, but +1 on string formatting support.
There's an obvious benefit in providing the latter in order to get a better "feel" for the value of a rational number by displaying a Decimal approximation, but questionable benefit in providing the former.
Explicit conversion is sufficiently rare that I'm OK with the idea of either going via a string (as used to be necessary for binary floats) or by explicitly casting the numerator to Decimal, then dividing by the denominator (no need to cast them both).