Privacy and security are not the same thing, although they are related. The secret is that your data remains yours and is not disclosed or transmitted without your knowledge. Security concerns your computer or device so that it is not tampered with or contaminated.
Apple has created a system called Gatekeeper, which is designed to ensure that only secure software runs on your Mac.
Unfortunately, the makers of Gatekeeper used standard Internet protocols to implement this security feature, meaning they chose to send data without encryption.
Not only is data sent unencrypted, but Gatekeeper intentionally bypasses VPNs, which means you can't hide your activity even if you try too hard.
It is difficult to say how many users it affects, but it is possible over 100 million. Apple CEO Tim Cook announced that there were more than 100 million active Macs worldwide at the end of 2018. Apple sold nearly 18 million Macs in 2019, and possibly more than that in 2020, as laptop sales increased due to growth of work from home. All Mac owners running Mac OS X Catalina (released in 2019) or later use Gatekeeper, whether they know it or not.
The content of the leak is not particularly terrible: it is not your name, passwords, credit cards, biometrics or anything like that. It's a constant window into your daily behavior that most Mac users are unaware of.
In the US, it is illegal for Apple to keep your data confidential from the government if requested by the government. The US government does not ask for them very often: Apple's transparency report shows that from July to December 2019, the US requested data from Apple to its customers 5.271 times. The 2020 requests have not been notified yet.
In addition, Apple has promised to launch a new encryption protocol for authentication checks and a new arrangement for customers so that they can be exempt from Gatekeeper security protections.