I've found that Trekwood is extremely effective and efficient in obtaining parts (and making money by selling those parts). If they say Keystone doesn't have it, I'd be confident that they (Trekwood) has also checked other sources for the part. You might contact them (Trekwood) by phone to confirm that, but it's always been my experience that if it's available, they'll (Trekwood) will find it and ship it to you.
That said, the rock guard may no longer be "in the supplier system or available from Keystone"... That's the normal supply chain. If the part has been discontinued and all supplies have either been sold or auctioned off, it may be available in the "aftermarket supply chain". That consists of wholesale salvage RV parts houses, RV salvage (junkyard) businesses and even RV dealership service/parts businesses, any of which may have one or two "gathering dust on the shelf" in their "back warehouse".
Finding parts in the "aftermarket supply chain" is, at best, hit and miss. You might find one on the first inquiry, it may take you several months of searching and sending emails to "google search results" before finding the part or you may never find one.
Have you considered rebuilding or repairing your existing rock guard or locating something that would work but is not the OEM part? That may be your only option for this replacement.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS