Right now, deleting __debug__ is not prevented:

>>> def f():
...     del __debug__
... 

Of course actually executing it doesn't work:

>>> del __debug__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__debug__' is not defined


Compare with assigning to __debug__:

>>> def f():
...     __debug__ = 1
... 
  File "<stdin>", line 2
SyntaxError: cannot assign to __debug__

For comparison:

True = 0
SyntaxError: cannot assign to True
del True
SyntaxError: cannot delete True

It almost seems like __debug__ is a hidden keyword value, but with a startup-dependent value

What version are you using?  As far as I can test, it has become a syntax error since 3.10. Here is the result with the latest 3.11 alpha

> python
Python 3.11.0a2 (tags/v3.11.0a2:e2b4e4b, Nov  5 2021, 20:00:05) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> del __debug__
  File "<stdin>", line 1
SyntaxError: cannot delete __debug__
>>> exit()

> python
Python 3.11.0a2 (tags/v3.11.0a2:e2b4e4b, Nov  5 2021, 20:00:05) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> def f():
...    del __debug__
...
  File "<stdin>", line 2
SyntaxError: cannot delete __debug__

Verified in IDLE also.  Thanks for doing the test I should have done.

ouch, apologies for not checking that!