in general, you're absolutely right: existing links should keep on working even after bigger updates on a website. And all vital links on Wizzley are still fully in tact or are redirected correctly.
Redirecting each and every link is feasible for smaller websites, but it can get quite difficult for larger projects like Wizzley. It sure is possible, but it involves a larger portion of code, that is harder to maintain and work with in future updates. Also, those particular redirects involve rather resource consuming server operations. And we consider having a fast loading site one of the really important factors - both for readers and authors.
Just recently, Google has (again) removed several of their services and - since it fits quite well here, too - I'd like to quote one of their statements:
"Technology has the power to change people’s lives. But to make a difference, we need to carefully consider what to focus on, and make hard decisions about what we won’t pursue. This enables us to devote more time and resources giving you products you love, and making them better for you."
As far as I can tell, the mentioned category links are used realllllly rarely on the web - and thus, will not result in heaps of 404s. Google and other search engines were never allowed to index those URLs; even more so: they are not granted access to them, since they are blocked via robots.txt. If you can show us, however, that there are really a lot of such links pointing to the old URLs (that cannot be changed by you or us), we certainly will consider including those redirects.
An expert is a man who has made all the mistakes which can be made, in a narrow field.
Niels Bohr (1885-1962)