If you don't like the empty 'Filter by category' dropdown then you can get the regular dropdown back.
A member of the Facebook Advanced WooCommerce group asked about changing the category box in the Products admin area from a “Filter by category” ajax one to the basic “Select a category” dropdown. The change came when they upgraded from version 2.4.6 to 3.5.2 (that’s 3 years of changes!!).
One of the admins replied to explain that the ajax version is used when the number of product categories exceeds a certain threshold. He explained that the threshold could be increased via the ‘woocommerce_product_category_filter_threshold‘ filter and warned that it could result in longer loading times for the Products admin pages. He included a link to where the filter is used. The threshold is set to 100 categories.
Using the filter
The filter function is just passed the threshold number (100) so we can simply return a bigger number. I chose to return the maximum possible number – PHP_INT_MAX. This will guarantee that the static category dropdown will always be used. Obviously this could slow down the loading time for the Products admin page.
Before and After
I tried it out on a site that I maintain. Although this site only has 5 products it has over 100 categories from an earlier version of the site. This causes the dropdown to be changed to the ‘Filter by category‘ version.
After including the mini plugin above the dropdown returns.