Comparables are often relevant, and looking for contradictions in what people argue or believe is a tool for shedding light on a situation. Somehow the word "whataboutism" became catchy enough that people started to believe in the right to exclude unwanted examples this way, but that's clearly a fallacy. If you can show how I'm applying a principle selectively in different cases, you've taught me something valuable. We need to stay open to that.
There's a subtler fallacy worth pointing out too: it doesn't follow that you're justifying the thing I'm against. That is, if you point out an inconsistency or some comparable example on my side of the argument, it doesn't follow that you're a communist or whatnot. Maybe you are, maybe not. Maybe we actually agree. All that follows is that you've found an inconsistency in me, and that's information I need.
When someone brings up an example that they think is relevant and you don't, the way to refute that is not with a garlic word, but to explain what the significant difference is. If there isn't a significant difference, maybe it's not irrelevant.
People do bring up irrelevancies, but that's rarer than it seems. More likely it's that they have a different perspective on what's relevant, as part of the original disagreement that needs hashing out in the first place. Cordoning off discussion doesn't help with that, and no party to a dispute gets the power to draw the lines of what's in and out for everybody. Indeed, that would amount to the power to impose one's view on the entire dispute.
And I wonder: if we could agree on what's relevant, perhaps by including everything we both really care about, how close that might get us to resolving our conflict itself.