Data is simultaneously Google's greatest strength and its biggest liability. If an employee uploads data to a not-known-to-be-secure third party, it can literally cost Google billions of dollars depending on the data locality. Or, it could plausibly cause a severe security breach (if e.g. an employee uploads a security token into a third party decoder).
And so the security team builds an extension in order to very loudly alert employees when they're on one of these known-to-be dangerous domains. And, as it happens, the implementation of the loud popups could make it a general browser notification platform whenever an employee visits a particular domain.
Is it a good idea for teams to start using it as a general notification platform?
No, not on any level. On top of being annoying for employees, it breaks the security UX. When someone sees one of these popups, they know they're important and that they should be very wary of whatever they're doing. But if it starts being a popup that can be triggered by anyone in the company who can convince themselves that they're providing value by sending out the popup, people start ignoring it. And there's no going back at that point, because employees have been trained to ignore the popup because of it staying stupid shit like "Happy Birthday Sundar!" and "Read these NLRB rights!" and "You can go grab a burger at the cafe today!" Everyone's been made worse off, and the original security problem has come back.