Shopify provides the list of filters, and the theme displays them. It’s now up to Shopify to build the functionality needed by merchants.
The good news is that we know Shopify is working on a new way of filtering by user-defined categories, and with this update, they will be able to drop this in whenever it is ready and it will instantly work in all OS2 themes.
Unfortunately, we don’t have any more information about this.
We are nudging Shopify about this daily through our private channels, as we know this is a concern for many merchants, and you are not the only one to ask about this.
At the moment, you will have to use the availability/price/vendor/type/variant filtering, and you can contact Shopify’s support team directly about this if you want to apply more pressure on them.