Well that’s straightforward to identify, take a look at the source code, it’s very fundamental to the design:
It’s a major functional pillar.
This single integer attribute is all over the model code, API, back end processes and UI. Assumption that there is only one in dozens of Theme Components and Plugins too, I imagine.
You’d have to be very brave to try and change such a core design/assumption and it would be a lot of work, and you’d probably not be able to use many of the extensions available in the ecosystem.
You’d almost certainly find it a lot of work/economically impossible to ever upgrade the instance, too.
Can I suggest you use tags instead of a category?
A topic can have many tags and so can appear in many tag lists.