Clicking on a link from a Google search, I was repeatedly asked to confirm that I’m not a robot (but with no actual Captcha tests). The fourth time, a warning showed up, telling me that I had been infected, and I must call MacAfee support to correct it. I closed the tab and the search, but now I’m getting notifications to click on to renew subscriptions to MacAfee, Norton, and even Apple antivirus apps.
I shut down the MacBook and restarted, but the notifications continued. I ignored them all, of course.
I haven’t gotten another since I started writing this OP, but can somebody clue me in on what’s been going on?
Sounds like you picked up a virus. If you have time machine, I’d restore to a day or two before you noticed the problem. If you don’t, I’d try running a known anti virus package. (Obviously not one of the ones you are being prompted to click on.)
I am doubtful that it’s a virus. Common these days is a misuse of the notifications system in the Safafi browser. Questionable sites barrage you with a bunch of confusing messages, one of which tricks you into accepting ongoing alerts from a particular site. This Apple support link discusses a similar sounding McAfee issue, and how to remove the notifications.
It doesn’t help that they name their website notifications something that looks like something you should not delete, like “Notifications” in the example below and not “McAfee Spam Site”.