WetGeek So I need to wait until tomorrow morning to look there.
I think I may have found the solution, but I won't know for sure for a while. On the media machine, I opened the menu and entered Notifications in the search field. A list of four items was shown. At the bottom of that list, was an entry that I didn't quite understand, but it contained Vivaldi in it. I clicked on it, expecting to see more of a clue, but instead the menu disappeared.
I repeated that step, typing Notifications in the menu's search field again, This time a list with three entries in it was displayed, The one containing Vivaldi at the bottom was missing. I'm pretty sure the menu was listing sources of notifications, and I had removed Vivaldi as a source. As we watch TV starting today, if I don't see any notifications from NewsMax, that will confirm my suspicions.
Being a glutton for punishment, I entered Notifications once more into the menu search field, and two items appeared in the list. (I'm pretty sure the first item was just a caption.) When I clicked on that second item, which was now at the bottom of the list, the System Settings Notifications settings dialog appeared.
I tried the same thing on my laptop, and no list was displayed. It simply opened the System Setting for Notifications. That sounds about right. I don't think I ever approved seeing notifications from any non-Plasma source on my laptop.