I have the same problem- I need to have Wifi Calling on because of the lack of cellular signal at my house, but damn if that notification isn't crazy annoying! I could not find anything either on how to disable it, the only thing that I did figure out was how to "mute" it in a sense. When the notification pops up, I long press on it, which usually gives you a few options for notifications, one of which is to turn the notification off. In this case, you are not allowed and it tells you, as you already surmised. However, a "Details" link pops up and if you tap on that, it takes you to Wi-Fi Calling settings, which is no longer in the Communications section of settings... Anyways, only two settings that are listed, top one being "App Icon Badges" and the lower one is "Wi-Fi Calling" which is enabled and does not allow you to disable, but if you tap on that you can adjust the notifications for Wi-Fi Calling and I played with the first setting which is Notification Style. When you tap on it it expands a menu which has "Sound and pop-up, Sound, Silent and Silent and Minimized". I chose "Silent and Minimized" and have not seen a notification since, unless I go back in and change it back, which I just did now to write this how-to. Hope this helps...
I have always been eager when it comes to updates to the Android OS, but this one for PIE has left me quite unhappy with all of the changes, especially when they design annoying notifications that they don't even allow you to disable... Anyways, I was glad to find a work-around and hopefully it meets your needs!
Manfred