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I think the changeset which made decoders to use _PyUnicodeWriter (issue16311) 
is responsible of the regression.
For example consider b'\x80abc'.decode('utf-8', 'backslashreplace').
The writer reserves string buffer with size 4 (every byte produces at most 1 
character). First byte is incorrect and replaced by 4-character string 
'\\x80'. The writer increases min_length but doesn't resize the buffer because 
its size is enough to write replacement string. But following writes of ASCII 
characters cause buffer overflow.