Earlier quoted context omitted.
>There's no way for me, a hypothetical person who wants to keep my Signal messages 100% encrypted in the Signal ecosystem, to opt out of my contacts using an Element bridge. This sounds like a made-up concern. How do you stop me from screenshotting your convo and posting on twitter, or just copying and pasting from one app to the next?
Those are distinct from my concern. In your scenarios, I have trusted you to keep the conversation secret, and you betrayed that trust. My concern is about giving messages in an automatic fashion to a third party, which I'm completely unaware of and have no way of making an informed decision about. The third party could be breached (they run an online service, which is much, much easier to attack than some dude's iPh…
An Element bridge is just a Signal client hosted on Element's infrastructure. Them using an Element bridge is no different than them using an extra device you didn't know about. That device could've well been insecure, or shared by many people, or hosted in the cloud. If you care about this, you should ask.