"necessary that" is correct and usable in written English.
You can use it to introduce a clause that expresses a logical consequence or need. Example: It is necessary that we take action on this issue soon.
How necessary that will be today, we'll have to wait and see".
And recently, I was reminded once of again of how necessary that is.
Is it really necessary that your puppets talk?
It is absolutely necessary that I acknowledge this other family.
It's just not necessary that anyone be misdiagnosed anymore".
"But it is absolutely necessary that we do this".
It is also necessary that rewards and punishments are included.
"It's necessary that stress tests are credible," Mr. King said.
It's not necessary that I repeat it to the media.
It is very necessary that industry should be nationalized, but it is more urgently necessary that such monstrosities as butlers and 'private incomes' should disappear forthwith".
Secrets hold out the promise, false but necessary, that death will be deferred until their unveiling.
