In this case, when implementing this extension on the latest magento version 2.3.6-p1 and with PHP 7.3 the rewrite function does return the CookieMetadata as the parent function does: vendor/magento/framework/Stdlib/Cookie/CookieMetadata.php:157
In this commit the correct parameter type is added as well as the correct return type
Ideally this needs to be a new bump from 2.3.1 to 2.3.2
In this case, when implementing this extension on the latest magento version 2.3.6-p1 and with PHP 7.3 the rewrite function does return the CookieMetadata as the parent function does: vendor/magento/framework/Stdlib/Cookie/CookieMetadata.php:157
In this commit the correct parameter type is added as well as the correct return type
Ideally this needs to be a new bump from 2.3.1 to 2.3.2
Please edit my target branch if it's incorrect.