From php 8.0.9 Creating/modifying with square bracket syntax $GLOBALS https://www.php.net/manual/en/language.types.array.php#language.types.array.syntax.modifying otherwise fatal error.
The isset function checks initialization and if not null of a variable, I think the change in reading has changed the behavior for unset on GLOBALS, yes it's a bug.

But it is good programming practice to know the available predefined types of php and the types of your code even if you are not sure to find the variable, otherwise the code change is synonymous with something that the programmer did not foresee .. It can mean hacker attack or more simply code modified by third parties (also the personalization of the php source and which does not coincide with the php team).

I think with this short piece of code it helps to understand, Rightly $GLOBALS, $_POST can be multidimensional or monodimensional arrays and therefore if you apply the fatal error in the writing phase it must coincide with all the variables that are arrays.
So this is a request not to show warnings on unset $GLOBALS even if the key/index does not exist or the variable is non-existent. Also, the features go to all types of array returns

Note* $_POST = null and valid context for array "function valid_superglobal()"

$var = array(array(null));
$_POST = null;
//if $_GET, $_POST, $_COOKIE, $_FILES, $_SERVER, $GLOBALS not multidimensional,
// or monodimensional array is bad code

$string = 'not valid';
if(valid_superglobal())
$string = 'valid';
unset($GLOBALS['var'][0]);
echo $string;
//delete all
unset($GLOBALS['var']);


function valid_superglobal() {
$result = false;
$return = false;
if(isset($GLOBALS) === true)
if(is_array($GLOBALS) === true) {
$return = true;
$result = array_key_exists('var', $GLOBALS);
if($result === true)
$result = array_key_exists(0, $GLOBALS['var']);
}
if($result !== true) {
unset($GLOBALS['var']);
$GLOBALS['var'] = null;
unset($GLOBALS['var']);
$GLOBALS['var'] = array(0 => null);
}

return $return;
}

This is caused by the implementation of
<https://wiki.php.net/rfc/restrict_globals_usage>.  Not sure if
that is deliberate.  Nikita?