Is my iPhone 11 pro max still waterproof after a small crack

That’s hard to say. Water can definitely squeeze through the cracks and ultimately find something to corrode. That will depends on just how much water and how long it is exposed to water.

However, it should be pointed out that the phone was never waterproof to begin with, just water resistant (IP68). It was never meant to be “used” in wet conditions, just resistant to those conditions. They certainly don’t warranty it if there is water ingress which tells you how confident they feel about it.

If you want peace of mind, then I would bring it to Apple, they will do a full replacement but it’s expensive and you will be getting a refurbished device in return. Otherwise, keep it as-is for now and save some dough and do your best to avoid water.