All ways of assigning to __debug__ are forbidden:

>>> __debug__ = 1
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> for __debug__ in []: pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> with cm() as __debug__: pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> class __debug__: pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> def __debug__(): pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> def foo(__debug__): pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> import __debug__
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__

The only exception is the assignment operator.

>>> (__debug__ := 'spam')
'spam'
>>> globals()['__debug__']
'spam'

This looks like a bug.