Someone could post a thread with an interesting premise, and the first response could be $racial_slur, and then the hivemind would repeat the process and spam $racial_slur indefinitely without any kind of real discussion going on. For the lulz.
Right now on /leftypol/ there aren't many threads of what you describe. They do exist, but they aren't numerous. What you describe is more similar to 4chan's /b/, but even 4chan's /g/ has "tightened up" and is being steered more toward discussion (still with racial slurs but not entirely focussed on them). A notable exception is the "DESIGNATED SHITTING STREETS"/ "POO IN LOO" meme, though.
I don't know if these are just my experiences.
/p/, /trv/ - slow boards that have their own shitposts.