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So the favoured of tzeentch rule that all thousand sons formations get says the following:

"If a war cabal contains the maximum number of units, then all units from the formation can re-roll any failed saving throws of 1".

So my question is , if one unit from the formation dies, does the formation lose the ability to re-roll failed saves of 1?
As the formation no longer "contains the maximum number of units".

Or does the rule mean that if you start the game with a full formation, you get the rule for the whole game?

The formation is written on your list. Your army is on the battlefield. Your list doesn't change if a unit dies. You keep the rerolls to the last man.

Completely software design-wise, in a different case the wording would be "While the war cabal contains the maximum amount of units..." which would suggest a continuous situation that would end when the clause of the "while" would cease to function. A wording of "If...then..." is usually taken into account instantaneously.

If you lost the re-roll it would tell you, like the daemonic tetrad does: this loses bonuses per DP lost. As it is you keep the re-roll.

Ah good.

I was hoping that would be the answer, otherwise the rule would be pretty useless.