Maybe I’m mistaken about what that topic was about - I thought it was about having the category show up in title and url of the category pages.
If it’s about having the category name in the topic urls then that’s a different issue altogether - not an entirely bad idea to add it but might make for very long URLs, e..g:
https://meta.discourse.org/t/features-category-name-in-url/29113
A slash between category and topic title would probably be more google friendly but would probably wreak havoc with the way urls are handled within discourse.
https://meta.discourse.org/t/features/category-name-in-url/29113
Category name does show up in topic titles as you said.