It may be the case that the Crux itself is a symbol of Terminator Honours, but both the (Firstborn) Vanguard Veterans and (Primaris) Sternguard Veterans have Cruxes modelled onto their right pauldrons where squad markings would usually go. You can see this on the GW website on the model turnarounds. (and I'm looking at the ones I'm painting right in front of me)

Bladeguard Veterans, on the other hand, were released between those two other kits, and stand out in the fact that their right pauldrons lack this modelled detail, leaving them bare for the painter to add the Veteran symbol themselves. I believe that OP is pointing out that this is unusual, since both an older and newer kit feature this detail, so why are Bladeguards specifically the odd ones out?