Cortana made Chief’s Halo 4 design, and this is true. For a while it was completely unique, and is still iconic. But people have missed the comic where Chief and Blue Team encounter the Didact on Installation 03. The Didact crushed Chief’s helmet, and after the skirmish his armor is modified by the UNSC (which is why his helmet and chest gash were repaired by Halo 5.) While the design wasn’t invented by the UNSC, they have worked on it, so it isn’t entirely alien to them.
Hopefully it’ll be put into the UNSC’s armor requisitions later on.
centurion matches the pauldrons almost perfectly and part of the chest plate …reaper i feel is closes to the chest plate of the chief…tbh the whole problem would be fixed if we have more degrees of freedom for customization like in reach…that was the most versatile version and you could make an immense number of combinations with the far lower number of pieces as in halo 5.