You are correct - it's under the general help for Privacy:
https://www.wikitree.com/wiki/Help:Privacy
It says under the description for Open Privacy:
- Profiles of non-living people without any sensitive information or real privacy concerns should be Open.
- Profiles of people born over 150 years ago or who died over 100 years ago must be Open.
I don't believe it happens "automatically", but I believe there is a process in place for a report to be run, profiles to be identified, and a script to run that cleans these up periodically. So it might not happen the day they go past one of these threshholds, but I believe it happens soon after.