Paramount-Paranoia but will automatically assume you're a minor if you do not verify your age and thus will scan all messages sent by you.
And they did not do that already? It's not an end to end encrypted service and I'm assuming that all interactions by users are logged and available to Discord employees. But it's strictly speaking not relevant to the OP's question; all the messages you send in the GrapheneOS chat rooms are already public.
As I've said, there is no indication that accessing the GrapheneOS chat rooms will require age verification.