....Separately, I have a hard time imagining such a small revision to the fender would make any difference in how the Mustang would absorb or displace the energy from a front-end collision or help keep the driver safe(r). Nothing else changed in that regard. .......
I can testify that yes it changed how a fender might crush and in some cases become a spear of types that could go through the windshield towards the driver and instead in many situations create a somewhat reliable or predictable result due to the features.
Similar design features were also included in the design of the inner fender panels also if you want to look at those.
Do we know if these triangles appeared on the front fenders of any other Ford or Mercury vehicles at the same time, such as the Falcon or Fairlane, or is it unique to the Mustang?
Looking randomly through some of my pictures of other years and models I can report mixed results. Its possible that with other designs of fenders and front ends the engineers addressed the concern differently for different models and years. If you notice there are two reliefs in the forward drivers side inner fender panel during 65-66 but an additional one placed behind the shock tower and the V shapes disappeared from the fender designed starting in 1967 Mustangs. Design change suggests to me that the engineers address a requirement initially with the fender change and then later addressed it differently with the model change. For additional views you can go to Ebay and look at multiple years and models
Just some of the cars I pulled up quickly
Raised V's
66 Fairlanes Some not all similar to Mustangs
70-71 Ranchero & Torino
72 Ranchero & Torino though the crumple feature is vertical due top fender design
No Visible Raised V's on Fender
66 Fairlanes Some not all similar to Mustangs
67 Fairlane
68-69 Ranchero & Torino though there may be reliefs of a different design
67-70 Mustangs
71-73 Mustang and Cougar though I believe like Torinos listed above there is a different style relief. Will have to double check when I go out the garage and open a hood
Wondering out-loud - Do you have another explanation for the V's?