Thank you, Porthos, so much. I thought I had a much bigger problem. I went through the steps in that article and actually found a site named shortcut.at or something like that and was allowing notifications from it. I've now blocked the site. I think I did some serious damage to my computer because another software, Bitdefender, I think, found a virus in all my TimeMachine backup files but was taking days to delete them, reporting that they could be neither sequestered nor deleted, so I manually deleted them. And now I can't get them out of the trash either by restoring them or permanently deleting them and I'm getting reports there's not enough room for another Time Machine backup. My guess is, it's going to have to reinstall the OS to fix this one. Criminey. And all because of a notification.
But again, thanks for referring me to that article. It has saved me a LOT of worry and additional aggravation.