Miner's Lung

a Forge mod adding hardcore air mechanics.

Below Y=0 and above Y=128, and in the Nether, you slowly lose air.
In the End, there's no air at all.
You can protect yourself with Potions of Free Breathing (which were renamed from Water Breathing), or with Respirators and Air Bladders.
Create little pockets of tepid safety for yourself with Soul Fire, Soul Torches, or Soul Campfires. Or, restore your lungs with a Bottle of Soulfire.
Safety Lanterns change color in your inventory and in the world to tell you the air quality around you. Or dye them for decoration!
Drowned will choke the air out of you on attacking
And as a fun bonus, right-click a torch to make it spew particles, perhaps to signal the exit to a cave.

All of these features are configurable: you can change the air quality at different levels in any dimension, and disable any of the mechanics.