They have been offering these vouchers for over five years (long before the flights moved to and from LHR T1, though when I say offering in recent times you had to ASK for the "Steimatsky vouchers". The last time we got them was at the end of March 2016 though, as I said before, we have only travelled Y since then until my wife and son about a week ago.
As they were only offered, recently, if you asked for them if you knew nothing you missed out. So maybe the C vouchers have got enhanced away since last March before many of you even knew about them or maybe they are still there but few at checkin know about it.
Maybe the TLV regulars, when they read this post, will have more information. I did send an email to customer services for information but am not holding my breadth for a useful reply.