WordPress at a glance
filter is not described

illegal_user_logins filter-hook . WP 4.4.0

Filters the list of blacklisted usernames.

Usage

add_filter( 'illegal_user_logins', 'filter_function_name_4005' );
function filter_function_name_4005( $usernames ){
	// filter...

	return $usernames;
}
$usernames(array)
Array of blacklisted usernames.

Where the hook is called

wp_insert_user()
edit_user()
wpmu_validate_user_signup()
WP_REST_Users_Controller::check_username()
register_new_user()
wp-includes/user.php 1504
$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
wp-admin/includes/user.php 167
$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
wp-includes/ms-functions.php 464
$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php 1119
$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
wp-includes/user.php 2366
$illegal_user_logins = array_map( 'strtolower', (array) apply_filters( 'illegal_user_logins', array() ) );

Where the hook is used (in WP core)

Does not used.