You have to specifically click a button to use it. That's a good compromise between jumping through hoops to enable it if opt-in.

In Signal is hidden under the post button, you can easily ignore it. In Wire it's on a toolbar and clutters the UI.

I think it would be acceptable if these types of things were on by default, and have the option to disable it or opt-out.

Yeah perhaps, but that would complicate the app development quite a bit. I'd rather see company invest developer time in other ways - maybe bringing desktop client support.