I'm wondering if there is a word that describes how it is essential that something not be true.
For example, where one might say:
"The President knows it is essential that we make it to the moon first."
The opposite would be
"The President knows it is [...] that the Russians make it to the moon first."
Of course, the negation "unessential/inessential" doesn't mean that, it just means that it doesn't matter.
The whole sentence could be simplified to "The President knows that the Russians must not make it to the moon first." but that's not used like the word essential.
You could possibly replace it with something more emotional like "The President knows that it is inconceivable that the Russians make it to the moon first." But that's not a real truth statement.