Can I make a substitute for milk in butternut squash soup for a vegan?

Buttermilk has a sour tang, so I might add some lemon juice to taste with whatever you decide.

Coconut is the defacto pairing for almost every South Indian recipe that involves a member of the pumpkin / squash family, I would definitely opt for that!

I make a lot of squash soups without any milk or milk substitutes at all. You might try what you're making without any additions and see how you like it.

You don't need dairy at all - what I do with "creamy" tomato basil is what I found on serious eats, and I think it could be translated to this use as well. At the very end, when you'd add dairy, instead add two slices of bread (cut into cubes, crusts off) and 2 tbsp olive oil, and emulsify.

Any nut milk would work. I love coconut milk in cream soups and coconut and butternut squash are nice together. When I say coconut milk, I mean the good stuff in the cans, not the chemical laden product in cartons.