Through the direct item search we will not get the expected result.
So we can create an item search and category search in NetSuite.
Item search:
Criteria can be blank , select name under the result.
From this search we will get all items in NetSuite
Commerce category search:
Criteria can be blank, Select item name/number under the result tab.
From this search, we will get all the commerce categories and items under each category.
Export the result of both into excel sheets and do vlookup in a new column in the commerce category file by comparing the names.
In the new column, use filter to filter out those N/A values, which indicates items not in the commerce category.