Yes. Effectively... nLockTime is a bit like a "
Dead Man's Switch".
You setup a transaction, that is not valid until X time (X = block number). Any time before X, you can invalidate that transaction simply by spending one of the inputs in a different transaction. If no action is taken, then the original transaction can be broadcast as a valid transaction after X has occurred.
GreenAddress uses nLockTime as a fail-safe for it's MultiSig wallets... They create a (partially) signed transaction (with their key) that sends all your funds to a nominated address from your 2-of-2 wallet with an nLockTime calculated to be around 90 days in the future. This way, if GreenAddress service goes down, you can simply sign the transaction with your key (so it is fully signed), and then broadcast it to recover your funds in 90 days.
But is it somewhere documented that it cannot be broadcasted by bitcoin nodes?
I don't think there is anything specifically documented that it cannot be broadcast... I think it's more just that because the transaction is considered "invalid" due to the lockTime that nodes will treat it like any other "invalid" transaction (bad signature, "missing" inputs, zero fee etc) and reject it (and thus refuse to broadcast/relay it).
My question was after I create and sign this transaction, what do I do with it?
All you can do is store it somewhere... most nodes won't accept invalid transactions, so attempting to broadcast it likely won't work. Why would a node want to store an "invalid" transaction in it's mempool? There is no point.
It isn't "unsafe" to have it stored somewhere (email, cloud storage, harddrive, given to recipient etc.) because, as already mentioned, if you change your mind you can simply invalidate it by spending one of the inputs in a transaction prior to nLockTime, and no-one can alter the "signed" transaction.
