The US OH-58D is currently missing the AN/AAR-57 Common Missile Warning System that it received late in its service life.
I think it should get it as, well, there is no reason for it not to have it, and it would be meaningless to have two different OH-58D variants with the only difference being this sensor.
It would also improve its survivability, especially compared to contemporary helicopters at the same BR, in exchange for having a very limited loadout.
NOTE: this only applies to the US OH-58D, as the Taiwanese one did not receive the AN/AAR-57 (as far as I know), and therefore should not have it
Brochure from BAE on AN/AAR-57: