In addition to flaking, people are highly reluctant to commit. Which is to say, they won’t say yes or no until the last minute. This is just as rude as flaking, given that many occasions require headcount for planning.
I've the feeling that this is actually a good thing, and that most flacking results from the fact that people feel pressed to say "yes" while actually they would not, and is just simpler to say "oh sorry" later. And since in US AFAIK people have hard time to just say "no" in general, they say "maybe" when they actually mean "no". They should just say "no" often and when they say "yes" do the best to attend since they really wanted to do it. But still "maybe" it's better than saying "yes" to something you don't like to do actually.
If the last minute wasn't good enough then it wouldn't be the last minute. If someone needs a headcount by a certain time, then state so and then that's the last minute - not five minutes after getting a text message.
It’s typically implicit given the nature of the event.