Some Mac users are being pestered by notifications to upgrade to High Sierra.
As I reported here just over a week ago, Apple installed this nudge using its pushed security and macOS patch service, on Macs which had not yet been upgraded to High Sierra. If you see this notification, it will reappear each week until you finally do upgrade, or it gets fed up some time in the New Year.
If you’d rather remove it, all you have to do is open the folder at /Library/Bundles, and trash the single bundle there named OSXNotification.bundle. As Apple created this folder specially for this bundle, there seems no harm in removing the entire Bundles folder.
(Thanks to Craig Grannell for alerting me to the fact that this notification has now activated itself.)