I'm working on updating my moon dimension in my mod and I'm trying to make it so there's no air and that you'd need a space suit, I can easily write the space suit part but I don't know how to remove the air.

Try to make an event that chek if the payer has the complete suit on them. If not they are hurted by a new custom damagesource :"out of air" or something like that
I did something similar with my radioactive ore.

Well, I would

1. hook into an event

2. check if entity/player is in the dimension

3. check equipment slots

4. if not wearing space suit, then damage player

you can then make a custom dmg source as mentioned above.