dpkg-reconfigure sysvinit
I don't remember what I was trying to achieve, but it was a bad idea. I also didn't (and still don't) know how to fix the outcome of this, so - since my home was on a separate partition anyway - I just reinstalled Debian since that was much quicker anyway.
In the vast overwhelming amount of cases tooltips show additional information that you cannot see from clicking on something or provide an explanation to an option that isn't available without scrounging through a manual. None of those apply here.