Even though I like the idea I see a problem with relying on getattr(numerator, "as_integer_ratio", None).
This assumes that all objects with as_integer_ratio will behave exactly like float and Decimal. If they don't we'll get inconsistent Fractions, or even errors. At least float and Decimal are "within Python's control" and we can guarantee the behavior.

Also, I don't see too much overhead in converting those objects to float and use them to create a Fraction. This would be a stronger assumption, IHMO. Especially if we consider objects that implement the same functionality but with a different method name, we'd have to convert them to float anyways.

Well, whether we rely on the method by the same name doing the same thing is mostly a philosophical question. Traditionally, in the Python community, the response was clear: if it quacks like a duck, it is a duck.
Especially in this case I do not see problems: the method name is very specific. If it was a rather generic name, say start, that would be a different question, but as_integer_ratio is pretty concise.

Just converting to float is simply not an option: fractions simply are much more precise than floats, as in exact. If you convert 1/3 to floats, you lose precision. Same happens for converting Decimals, or numpy's float128.

This all looks good. Please update the docs as well (including a versionchanged note).

After reading a lot on this topic on bpo, I realized that it could be a good idea to simply add as_integer_ratio to numbers.Rational. This makes everything very systematic, and also kindof baptises as_integer_ratio as the way to go.

How do you like it?

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Accepting any object with as_integer_ratio was rejected in #82017.