I haven't created a new shop yet, because it hasn't been my top priority at this point. Do I have to in order to get Quickstock back?

If you haven't deleted your other shops or galleries their quickstocks should work fine, although the links to their quickstocks have been removed.

Go to https://subeta.net/myshop.php?shopid=XXXXXX&act=quickstock after swapping the XXXXXX for the ID of one of your shops. Then bookmark it. :D

I see. Thank you!