This worked for me: long-pressed on the “Alexa Accessories” notification until the “i” in a circle appeared (this is the notification’s info settings area). I opened those settings by pressing the "i", switched on the “Block” option to block notifications from Alexa, then switched it off. DONE. I switched it back on and the notification still didn't' show up, but I don't really need any notifications from Alexa so I switched it back off, just to make sure it would not pop up again in my notifications.
Excellent advice! I did just as you suggested, and finally that annoying notification is gone! Can't thank you enough!