Are you talking like Halo 3 where you went around and picked up overshields and other power ups?
If so, that system was replaced by the re-installation of the health system from Halo: CE where shields and health were separate. In Halo 3 if you'll notice, there is no health bar, its combined with the shields. The only playlist where this is still present
is in Team Classic. Otherwise you'll have to stick to custom games to use it.