Do you have any apps that fuss with your network connection? Like one of those all-in-one tools like Clean Master?
That's most definitely not something that comes on a Nexus stock... and has to be an outside app popping that toast message... which stinks. Toast pop-ups are a pain to find the source. A system notification can be isolated by long pressing on the notification in the shade... toasts are in and out and you can't do anything.
I'm seeing this on a Pixel XL as well. It's a bit annoying and I'm uninterested in a factory reset. I have reason to believe it is coming from a 3rd party app, but I have no idea which one. Has anyone been able to figure it out?