PEP 526 explicitly states
"as with all dunder attributes, any undocummented use of __annotations__ is subject to breakage without warning"

I consider deleting __annotations__ to be undocumented use.

Do you really think that an obscure difference in the behaviour of 

>>> class C:
...     del __annotations__
...     x: int

justifies an extra bytecode, but the implicit return at the end of all functions (LOAD_CONST None; RETURN_VALUE) does not?