Super simple.
When viewing the list of Custom Forms in Setup, I want to add a column that shows where those Custom Forms are in use (or null if used nowhere).
This is similar in principle to viewing the list of fields, which has a list of forms on which those fields are used.
What is the syntax I can use to add the appropriate column(s) to the view?
As far as I know, there is no linkage in the backend that connects categories (custom forms) to their host objects. This is probably because the list could be massive and the query would take quite a while to execute as it would have to traverse every object in Workfront or each form would store every parent object ID.
Unfortunately, you can't even search for objects by associated category IDs, so if you wanted to build this yourself you would need to query each object and search its categories to see if your custom form appeared.