Getting weird Safari notifications that I think are scams - how to stop?

Hi! So I've been searching around about this for a while and have tried a number of things without any success and I'm kind of just lost on what to do now. Basically, every couple hours I will get a safari notification on the top right corner of my screen -- it looks just like an imessage notification, it comes up then disappears after a couple seconds or so. It's not exactly a pop-up, which is what I've been finding from most forums. And it will have something like "You have a new message!" or "You have (1) new security update!" or something along those lines. And it also uses the safari logo... it looks very convincing and just keeps coming up no matter what I try.

Some info about my macbook pro (I don't know how useful this is): it's macOS High Sierra Version 10.13.6 ; retina, 13 in, late 2013; processor 2.4 gHz Intel Core i5.

I have already tried a couple of things from other forums I've found. For one, I deleted all my website data and then restarted my computer. I also manually deleted my Safari data by going to ~/Library/Preferences and deleting "com.apple.Safari.plist". I also emptied all my caches using the Develop menu. I also (for a while now) have been using 2 adblockers. I don't know how any of these things help - I just followed advice from some forums I've found, and I don't know what else to try, so any help would be greatly appreciated!