Say, removing 4 Max Health if everything is below 50%, or what have you. Then gaining it back when the nutrient(s) go above the threshold.

Would it be possible to add this?

This already exists, just use negative numbers for the amount values.

The wiki says this:

amount - decimal

The amount to use for the operation on the attribute modifier

It doesn't say that the number has to be positive, otherwise I would have put positive decimal.

Interesting. I don't remember the wiki saying this, mind updating it?

Well, I just assumed it would kill itself if I used a negative number lol.

Still not used to 1.16 modding (been with 1.12 for years at this point) and bricking the game is not my idea of a fun time.