Turin,
Perhaps I can help.
Jet blast deflectors (JBDs) are still used with the F-35C, because like other USN jets, it launches with its exhaust pointing straight aft parallel with the deck (sometimes in reheat), and that severe and concentrated blast needs to be diverted upwards and away from aircraft and personnel in the vicinity of the catapults.
dat581 - remember that the USN carriers also have waist catapults, about half way down the deck.
The F-35B launches without a catapult, with its aft nozzle deflected onwards towards the deck. (On the QEC many of these launches will be about half way down the deck, not often right aft). The normal JBD design would not work with this, and moreover the launch position up a ski jump (or with a flat deck STO) is adjusted with launch weight and wind over deck (WOD), so a fixed JBD wouldn't be of much use.
Even so, the F-35 programme did look at other JBD designs and there were some that might have had some effect on the F-35B efflux, but early trials and further analysis showed that once the efflux hits the deck it spreads out and slows down very rapidly. Given this, the hazards can be adequately controlled by suitable precautions on where to stand during launches. The same goes for recoveries.
Hope this helps, best regards as ever to those who are going to work the decks,
Engines