Of course it's opt in, it'd be patently absurd for it not to be. Nobody would use a chat app where some kind of weird crypto payments were mandatory. Advertising its opt-in nature as a feature is like advertising your cereal as asbestos free.
I fail to see how, care to explain?
It's opt-in as any other feature which is not the core of the app. You don't have to send stickers or GIFs. If you don't want to receive messages at all, you shouldn't install Signal because it will be hard to avoid, it's a core feature, it's not really opt-in.
b) opt-in features that are very profitable have a funny habit of transmorphing in to opt-out