Yes toggle if implemented. I don't like shouts.... sometimes they become ways to make privatet beefs public.

Maybe have the toggle and two log-in buttons: "Log In" and "Log in SFW."

This might be splitting hairs, but "registered users over 18" as visibility option would be nice as well. I don't want kids looking at my account, even if they are registered. But yes, having the option to be unindexed and visible only to registered users would be great.