WordPress at a glance
filter is not described

username_exists filter-hook . WP 4.9.0

Filters whether the given username exists or not.

Usage

add_filter( 'username_exists', 'filter_function_name_3867', 10, 2 );
function filter_function_name_3867( $user_id, $username ){
	// filter...

	return $user_id;
}
$user_id(int/false)
The user's ID on success, and false on failure.
$username(string)
Username to check.

Where the hook is called

username_exists()
wp-includes/user.php 1343
return apply_filters( 'username_exists', $user_id, $username );

Where the hook is used (in WP core)

Does not used.