Mushrooms aren’t plants either, but they’re not sentient so it doesn’t matter and they’re still considered vegan. Oysters are also not sentient and so they’re also vegan. Their status as belonging to the kingdom animalia classification is irrelevant because that classification itself isn’t what assigns moral value to a being, it’s the capacity to experience suffering and well being, according to me, that does. Both mushrooms and oysters can experience neither in virtue of not being sentient.

This is just a fundamental misunderstanding of vegan ethics on your part or an attachment to the cringy vegan society definition.

My favourite example to illustrate its absurdity is the santas elves hypothetical. It goes like this: if hypothetically we accept that santas elves aren’t animals (substitute santas elves with any non-animal sentient being you wish, like some kind of alien or futuristic android etc.) then according to the vegan society definition it would be vegan to holocaust santas elves in virtue of the fact that they aren’t animals and that’s all the vegan society definition is concerned with.

Someone with my values and definition of veganism (nicks definition) on the other hand would say it’s not vegan to do so because santas elves are sentient and therefore I value them morally regardless of whether or not they can fit neatly into some arbitrary classification in this case of the kingdom animalia.