How is it possible to appeal against the suspension if they do not say which of the Twitter Rules is violated? "Platform manipulation and spam" seems the most likely, but how to be sure?
The goal is that it is not. After all you don't want the bad actors to circumvent the bans. Or trying to work at edge of them. Standard practise.
This is a good way to drive off good actors who get caught in the crossfire too.
It is not standard practice in criminal law.
What does any of this have to do with criminal law?
It's hard to avoid breaking the law when it is not clearly defined. Same can be said here.
Maybe that's why they write down the law, in books. On the whole I would say criminal law is fairly clearly defined, the parts that are more muddy are more muddy because lawyers are good at splitting hairs, not because the law was muddy in intent.
You're not going to be sure. Everything done is done arbitrarily.