Doesn’t matter if people think it should be a rule – it’s not going to become one.
Threadshitting is defined as coming into a thread and dismissing the topic itself, for instance by telling people they are morons to be discussing the topic. Thus it’s actually a very specific behavior.
OP: I like kitties.
Threadshitting: Y’all need to get a life. Enough of these freaking kitty threads. People who like kitties are assholes.
Not threadshitting: I hate kitties and prefer ferrets.
As long as you’re responding to the topic, and do so civilly, it’s not threadshitting.
Note the “civilly” – we can zap people for “being a jerk” as necessary. If someone wrote an OP about their mother dying and another person came in and started ranting about the Mom-centrism of U.S. society, or found an old post where the OP had talked about an argument with their mother, or something, I would probably respond as a mod – not for threadshitting, but for being a jerk.
Hope this helps.