Well, right now, a lot of people have their posts searchable already, and that shouldn't be a surprise, since there's an opt-out - that part wouldn't be a change, it would just use the existing setting.

(It would be nice to be able to easily see which journals have search indexing turned off, though. Maybe at least something in the profile? I can think of a couple reasons that would be useful to people. That might need its own suggestion.)