Problematic reading of a feed using simplepie lib

I have the following notice or error and it could be the reason why my feed isn’t coming in.

Note : IT WORKS ON LOCALHOST ! Thats the worst thing really as it confuses the issue for me and makes me think its an environment setting. But I am quite new to server stuff in a sense.

Here is the stack :

Notice: rss-functions.php is deprecated since version 2.1! Use wp-includes/rss.php instead. in /var/www/deployments/new/wp-includes/functions.php on line 3782

Notice: rss.php is deprecated since version 3.0! Use wp-includes/class-simplepie.php instead. in /var/www/deployments/new/wp-includes/functions.php on line 3782
testing
Notice: Trying to get property of non-object in /var/www/deployments/new/wp-content/themes/light/template-parts/rss-reader.php on line 21

Notice: Trying to get property of non-object in /var/www/deployments/new/wp-content/themes/light/template-parts/rss-reader.php on line 22

Warning: array_slice() expects parameter 1 to be array, null given in /var/www/deployments/new/wp-content/themes/light/template-parts/rss-reader.php on line 22
There are no items to show.

This happens for this demo script that is on your website : https://codex.www.ads-software.com/Function_Reference/fetch_feed

fetch_feed is awesome and well done for having such a great dev api for us to use.

I would welcome suggestions as to how to debug it as well. To get this trace on the server I set error reporting to true on the server.