This is one of the major breaking changes that we deployed to our doc site.
Why
Your article URL associated with category and sub category URL, eg: YOUR_DOMAIN/category/article. What if you want same article on different category for better discover? That article will show on different URL and google consider this as duplicate content.
Also, the article link was getting longer and ugly.
Solution
We moved article to its own route and not binding with category url, eg: YOUR_DOMAIN/article-url and category route is now YOUR_DOMIAN/category/slug.
We created a 301 permanent redirect with your previous route and Google is catching up fast.
We believe this change will bring more SEO traffic to your site and gives you a vanity URL for every article you write.
New: Sub category icon
Now you can add icon or image to your sub category also.