For what it's worth, "premium SMS access" on my Pixel 2 is in Settings > Apps & notifications > Special app access.
I agree that it's one I would leave turned off, or at most at "ask". Set it to "allow" and any app that can send SMS can run up a large bill for you (and there are scams and malware that do just this).
If the message isn't one you were sending manually then you really want to find out which app is trying to send it (and get rid of that app). If it is one you are trying to send yourself, find out how much it will cost and whether there are any other strings attached before sending (e.g. you don't want to find that it subscribes you to something that will cost money in future - yes, I'm very cynical about premium SMS).