• When below Y=0 in the overworld, or in the Nether, you will slowly lose oxygen. Wear a respirator to keep from suffocating.
• When in the End, you need water breathing potions in order to breathe.
• Right-click a torch to make it spew particles, to act as a signal. This way you can distinguish torches used to illuminate a cave and torches used to mark the exit while mining. (Breaking it will just return the torch, so it won't clutter your inventory either.)

All these features are configurable, so you can remove the hardcore air mechanics if you like, or (for modpack makers) set different dimensions they function in.

Curseforge Link

Thanks to nojustgavin#8915 for some of the textures!