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The problem is that "surrogatepass" specific to utf-8 and there is no standard way to decode alone surrogates in utf-16.
>>> "\udc80\udc80".encode("utf-16", "surrogatepass").decode("utf-16", "surrogatepass")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'utf16' codec can't decode bytes in position 2-3: illegal encoding
With utf-32 this "works" only thanks to the bug -- utf-32 allows alone surrogates (issue #12892).
If the "surrogatepass" worked with utf-16 and utf-32, it would be natural to throw ValueError for other encodings.