Doesn't it come up in one of the side stories that the reason people don't talk about Outsiders is because some of them are in hibernation on earth, and talking about them causes them to wake up?
This is the premise of the Oblivion War, I think; not "on Earth" but locked-out for lack of summoning. We see it mostly in the short "Backup," reprinted in the
Side Jobs anthology. And there's a big ol' WoJ laying it out, quoted here:
https://wordof.jim-butcher.com/index.php/word-of-jim-woj-compilation/woj-on-harrys-mortal-allies/(search for "jimbutcher" -- it's the 1st of 8 hits. The linked original 404error's; I don't know if it still exists, but my cursory Google only finds references to it, no hit for the original).
It's about erasing all record and memory of certain beings, who need to be Summoned to be able to reach into the world or influence it.
I don't believe it was ever made explicit that these were specifically "Outsiders" ... but the circumstantial evidence sure suggests they are!