I believe I have a solution to this. At least, I haven't had the unwanted notification for an hour now, and it was showing up every 15 minutes or so since I let my S8 update this morning. Under settings, click on the blue Samsung account icon in the upper-right corner. Completely remove your Samsung account from the phone. It will give you dire warnings about the terrible consequences of removing your account. Unless you use one of the affected Samsung services, ignore it and remove the account. Fingers crossed that it stays gone.
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