The linked PR proposes implementing a new sakai.property (wsetup.defaultSiteVisibility), which controls the default value selected in the "Site Visibility" section of "Site Access", when creating a new site of any type. The new config option will default to true to preserve OOTB functionality.
NOTE: this config will only be taken into account for site types which you've defined as being able to change this "public" vs. "private" setting by the end user, via the existing sakai.property site.types.publicChangeable. The relevant OOTB defaults effectively make course sites always public, and does not give the user the option to change this value for course sites. Therefore, OOTB, this new property will only control the default selection for project site types. In order to test the effects on both course and project sites, you would have to explicitly set site.types.publicChangeable=course,project.