Calling whataboutism is not a way to shut down debate. It's a way to refocus debate on what is actually being debated. Whataboutism goes something like this:
Person A: "Political party X did this thing, and it's wrong."
Person B: "Well, political party Y did this other thing, and that's also wrong, so who are you to judge?"
Person B is engaging in whataboutism, because the debate is not about party Y doing that other thing, which may very well also be wrong. But the argument isn't about that; it's about the wrongness of what Party A did, which is why Person B's example is whataboutism and a red herring. As a clearer example:
Person A: "Suspect X is accused of murdering someone."
Person B: "Well, Suspect Y is accused of an unrelated armed robbery, and that's also wrong, so who are you to judge?"
Suspect Y's alleged armed robbery has fuck-all to do with the wrongness of whether or not Suspect A murdered someone, and to imply otherwise is a fallacy. Calling whataboutism isn't "shutting down debate." It's calling attention to someone deliberately trying to distract people from the original point by saying that someone else is worse, or that someone else has also done something else bad. We aren't talking about that other person; we're talking about the original argument. Both of them can be bad independently, but the badness of the distractor has nothing to do with the badness of the original point.
There's a whole article on it here.
