Where is that set? I've been a mod on subreddits for a decade and I'm not aware of any such setting. I just checked 'subreddit settings' for one of my subreddits, and I see nothing about defaulting to subreddit-only search (which is a setting I'd definitely like to enable!).

If it is I couldn't find the setting for it (didn't look too long), but I did discover the site will remember (per subreddit) if you checked the box or not the last time you searched, so that's nice I guess.