Legally, there absolutely is. Because by law, the messaging app operator also can't just publish the stuff you write in a chat. Even some disclaimer in the terms of service probably wouldn't work if people would generally assume the chat to be private.

And it also doesn't even matter because WhatsApp claims to be E2E-encrypted.

WhatsApp has end-to-end encryption

Meta claims WhatsApp is end-to-end encrypted.

It's up to you to trust Meta or not, but people who trust them do have an expectation of privacy.

That's irrelevant here because the OP is running the LLM on one of the ends, so it's decrypted that same as when you're reading the chat convo yourself.

It also misses the mark because you're talking about an eavesdropper intercepting messages and the OP is the receiver sharing the messages with a third party themself.