Marking events private in bulk

There is an addon tool called Set Privacy

It has a variety of options to choose what will be made Private (or Public)

It also allows a Custom Filter to be used to choose a subset of your Tree to be “Private”. For instace, you might start with a People category custom filter that uses the People Probably Alive filtering rule.

The following thread discusses using Tags for such filters. And using SuperTool scripts to set Privacy based on complex criteria: