Are you talking about the notification you see when you click the bell icon in the global navigation? If so, then that is from Announcements. You receive announcements from wikis which you have edited on in the last 90 days. So, ironically, if you hadn't created this thread, you would have stopped getting them after tomorrow. There is not built-in means to disable them. However, there is AnnouncementsIgnore which you can import for all legacy wikis via your global JS. I am not sure if that script works on UCP wikis yet. Even if it does, you would need to import the script via your global UCP JS and the import methods seem to not be working correctly on UCP right now.