Douglas MacMillan and Robert McMillan reporting for The Wall Street Journal that "A software glitch in the social site gave outside developers potential access to private Google+ profile data between 2015 and March 2018, when internal investigators discovered and fixed the issue, according to the documents and people briefed on the incident. A memo reviewed by the Journal prepared by Google’s legal and policy staff and shared with senior executives warned that disclosing the incident would likely trigger “immediate regulatory interest” and invite comparisons to Facebook’s leak of user information to data firm Cambridge Analytica."
They went on to say "The closure of Google+ is part of a broader review of privacy practices by Google that has determined the company needs tighter controls on several major products, the people said. In its announcement Monday, the company said it is curtailing the access it gives outside developers to user data on Android smartphones and Gmail."
As of today, October 10, 2018, Google+ is back online but its approach to security is far and away more restrictive to developers.