I’ve known people who chose not to eat mammals, birds, fish, decapods (lobsters, crabs, prawns), or cephalopod mollusks (e.g. octopus, squid); but who were okay with eating bivalve mollusks (clams, mussels, oysters) on the grounds that they did not have enough brain to experience pain.

I think those folks would be okay with eating jellyfish.

Rather than asking, “Is X vegan?” it might be more useful to ask, “What is person P trying to accomplish by ‘being vegan’? Is eating X in conflict with that?”