Why don't I receive any messages when subscribing to $SYS on "shared" Hubs?

When creating a “shared” Hub, your Devices connect to the same message broker as other customer’s Devices. Getting a global view of the broker’s activity (through $SYS topic subtree) is thus not relevant.

On a dedicated or high availability Hub, you are the only customer on the message brokers, you can access this topic subtree.