ouch. haven't thought of this.

OTOH, just plain io with your example:

with open('xy', 'wb') as f:
    f.write(y)

Traceback (most recent call last):
  File "<pyshell#29>", line 2, in <module>
    f.write(y)
BufferError: memoryview: underlying buffer is not C-contiguous

fails too and after all that's not too surprising.

In a sense, the old behavior was an artefact of silently copying the memoryview to bytes. You never used it *directly*.
But, yes, it is a change in (undocumented) behavior :(