None has ever been mentioned. I gave you the logic behind my claim. Jump-5 has been around for four centuries, jump-6 for one. And there are Vargr worlds around with TLs of 15. IMO it's flat out unreasonable to assume that jump-6 is still a military secret, much less jump-5. Yes, there is canonical text that expressly states that the X-boats are the fastest available means of communication, but there's also evidence that the navy has jump-6 couriers.
First off thanks for the replies aramis and Hans.
OK, so no memory reset required

(at least nothing major) On to the questions...
Not a military secret. Just a military/Imperial governing prerogative(?). They require J5 and J6, limiting the yards available to produce and maintain that level of response, so civilian desires go wanting.
I don't see that as so far a leap.
I've long long ago simplified/codified it for MTU. The Imperial Government (including IN, IISS, High Nobles, and MegaCorps) have exclusive access to TL15 shipyards. Some paramilitary forces (Imperial Incorporated Mercenaries) have access to TL14 and lesser shipyards. Civilians (primarily Merchants) are left to work with the TL12 and lesser shipyards. I picked the split based on Jump, Power, and Weapons mostly.
Certainly governments of individual worlds may very well build up to J6. But to what purpose? And wouldn't the Imperial Military contracts be much more lucrative? What I'm saying is what world government needs J6?
Would I as a world government want some private fleet of J6 couriers just to keep up on the latest news from Capital? No. Why would I? I can't think of a good reason for the expense.
Why would you in such a position?
I see no problem with keeping the new of the death restricted to the IN and High Nobles.
Some (even all) MegaCorps having J6 couriers I have no problem with. Smaller lines I don't see having or needing it. Any line is going to built on less than J5 imo. I thought your own (or was it someone else) calculations showed the economics of it? Or (entirely possible) I misunderstood them.