Sentience is what I base my ethics on (i’m a sentientist or sentiocentrist), which has implications on diet when considering whether to exploit and/or kill sentient beings for food. I don’t think it’s arbitrary, if someone is sentient, they are morally relevant because they can experience positive and negative valence (pleasure/pain, to put it more plainly but lose some nuance). If something is not sentient, I don’t see how it can be ethically relevant except in cases where the nonsentient thing matters to a sentient being
if you’re looking for arbitrary, the anthropocentrists are that way
Also I agree we can’t prove that plants aren’t sentient, that’s why I said “to the best of my knowledge”