If it's to avoid all animal products no matter what, then it's not, because it's an animal product.
If it's to avoid exploitation of animals, than it is, because it's given willingly.
There are definitely moments when I don't want to breastfeed, but I still do because I think it's best for the baby, it's free, and it requires no washing up of bottle parts. In those moments, I'm not being forced, but I'm not 10000% willing either. Still vegan at those times?
In that case, I'll try and answer: I think that for 99.999% of vegans the question wouldn't even occur to them. Veganism is about avoiding hurting animals for food, not about human breastfeeding practices. The question seems bizarre.
I've known a handful of folks who abstain from animal products for religious reasons. One or two did not breastfeed because they view any food produced by/in any body as tainted. Ital eating--but hyper restrictive. They never referred to their practices as veganism, but there is that "don't eat animal products" overlap.
I thought some vegans don't consume animal products because they think they are unhealthy, not because of the suffering associated with it, but like I said in other comments, I really don't know much about veganism, but am open to it.
So you still out of your own free will decide to give milk to your kid. It's not as if your husband pins you down and forces you to give milk to your kid... You might not enjoy it, but nothing is stopping you from not giving it.
That's why I said it matters what your criteria is.
To me, it is. You're not being forced or coerced, you're just not enthusiastic about what you chose.
A moral system that doesn't allow people to feed their children is broken.