WP_REST_Users_Controller::check_username

	public function check_username( $value, $request, $param ) {
		$username = (string) $value;

		if ( ! validate_username( $username ) ) {
			return new WP_Error(
				'rest_user_invalid_username',
				__( 'This username is invalid because it uses illegal characters. Please enter a valid username.' ),
				array( 'status' => 400 )
			);
		}

		/** This filter is documented in wp-includes/user.php */
		$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );

		if ( in_array( strtolower( $username ), array_map( 'strtolower', $illegal_logins ), true ) ) {
			return new WP_Error(
				'rest_user_invalid_username',
				__( 'Sorry, that username is not allowed.' ),
				array( 'status' => 400 )
			);
		}

		return $username;
	}