Meta key and value and filter and url

@variousfire sorry for the late response. Please explain a little more.

May I have the meta key to save the value of the location?
On order placement or edit/update product page, you want to save some meta value of location, please explain.

I have tried _stock_at_id, location-ID and tax_input[location][], they all are incorrect.
How did you try? If you tried it with get_post_meta so what was the ID? Order ID or Product ID? If it was product/post ID so are you sure about the simple product or variable product? Let me know so I can add the missing piece of puzzle for you.

On the products page, I would like to set the filter by location but I couldn’t find any available widget related to the stock locations.
Products page means shop page? Because single product page has just a dropdown with the location names, and filtration might will help you on shop page, am I right? This point is a little clear to me so I am adding it in my to-do list as a new feature request.

P.S. I am closing this ticket as it is a feature request, not a bug report but still I will continue discussion with you to work on it.