One more property I think is very relevant to the types of memes referred to by the article is that these memes want to spread at the exclusion of all others or any variation.
What stood out to me with respect to that event is that while many memes were thrown about, some memes had to be clarified to death without having any effect on the outrage while others continued to be thrown about without requiring any clarification despite being dog whistles for things significantly worse than their seemingly innocuous wording.
Accusing someone of supporting genocide should not be considered an appropriate way of asking them to provide a more considered, nuanced opinion.
And if you say that both sides have a lot of valid points and you aren't sure how to weigh all those points to reach a clean and simple conclusion? Well... I've never won any friends like that.
But to answer your more general question of who gets to decide "appropriate", that is society as a whole. We, as a society and as a civilization, decide what we find acceptable and objectionable. Right now we have drifted into a territory where a one or two sentence soundbite is considered appropriate to encapsulate complex issues that defy easy solutions despite the soundbite pundits, politicians, and activists that spout them as axiomatic truths, shutting down the ability to have a conversation with anyone who slightly disagrees.
Do you have more insight on this, on how it could be handled better? Or are you happy with the status quo where little conversation but much shouting is the norm?
Would "I'm standing with the people of Israel" count? Because it is a complete throwaway statement, made by US politicians so routinely that it is absent of all meaning. Israel is a democracy that upholds property rights and rule of law. Unfortunately, rule of law that successive civilian governments have put in place allow settlers to simply take, by decree, land that belongs to Palestinians. That's not 'property rights' it's Manifest Destiny.
Andrew Yang did not get cancelled. He propagated a meme statement and had to clarify that his actual opinion has more nuance, as does the situation he referred to.