Well, that looks concerning personally, even if it’s an opt-in feature. I don’t trust them, because they could be like “Hey! Look at this opt-in feature” to build some trust, and then turn it on by default in the future.
Thankfully, Facebook users do have an option to turn off camera roll cloud processing in their settings, which, once activated, will also start removing unpublished photos from the cloud after 30 days.
What about Instagram?