Utah Patch 10 (starting upgrade to WaDC in a couple of weeks). 

Catalog Item form - Related lists:

'Assigned Topics' is not available

I added it to the 'Record Producer' form with no issues

The name of the related list is "Connected Content -> Catalog Item". 

Open any existing catalog item from Maintain items table.

Click on Hamburger sign --> Configure --> Related Lists --> Select "Assigned Topics" and push it to right bucket. It will be visible in your Form view.

You can find as "Connected Content -> Catalog item"