It's no different from saying a gathering is going to be alcohol-free

It is completely different. Alcohol-free parties allow dry ex-alcoholics to remain unrecognized, which I think is a flawless moral motive. Inviting for "a vegetarian gathering" or "bring your own vegetarian food" is less well justified, but can be accepted.

The term "dead animals" amounts to a targeted insult, comparable to derogatory names for ethnic backgrounds or sexual orientations.