Yes, it is a health bar. The bar only depletes when the player takes damage after their shield is down, thus it must necessarily be a health bar. However, unlike in other Halo games with a visible health bar your health regenerates.

: If so, why would they bring back such a dated concept to Halo multiplayer?

ಠ_ಠ

Really? If you need it explained to you, here goes: There's always been health in Halo. That's why you don't die instantly when your shields are depleted. Health operates the way it did in Halo 2, 3, & 4. The HUD just has a widget for you to be able to monitor it.

I have been arguing for years in favor of a health bar regardless of whether or not health regenerates. How much damage you can take after your shields are down is need-to-know information, and the absence of a health bar to relay that info to you was a complaint I had about the HUD in the aforementioned games.