Health Regeneration in Call of Duty: Modern Warfare goes back to the auto-regen style that was present in the franchise prior to Call of Duty: Black Ops 4.
With health regeneration being automatic, players no longer have to worry about manually healing in-game anymore.
With this being said, however, there is a Tactical Equipment in the game called Stim. The description for the equipment is military stimulant that cauterizes combat wounds. And, it does just that — it speeds up the auto-health regeneration that is in the game already but just a few seconds overall.
Personally, I never ended up using it as I wanted to use a different Tactical Equipment, and based on the event, there was not many people complaining or even mentioning that Stim was a problem in Call of Duty: Modern Warfare.
Auto-regen is only available in the standard MP modes. As mentioned prior, Gunfight, the 2v2 mode in Modern Warfare, does not have any health regeneration unless the Stim equipment actually is in one of the pre-set load outs that you work your way through.