It's been declined https://github.com/LemmyNet/lemmy/issues/4225
Imagine the same news article is posted in /c/cars and /c/fuckcars. It is not desired nor expected to have all comments combined.
Making it a checkbox is an interesting idea though, but could still be confusing reading the comments with the different contexts.
I've been working in Ruby on Rails lately (unfortunately) and yeah it's extremely bad at this. There's so much hidden implicit behavior everywhere.