Disable iOS Software Update notifications?

At work we have several iPhones running old versions of iOS for testing our software. Since Apple does not allow downgrading it’s important that these phones do not get upgraded. Unfortunately Apple is very aggressive on iOS 13 about upgrading. We get nagging pop-ups to trigger us to upgrade to the most recent version of iOS (sometimes multiple pop-ups within 1 hour).

How can I prevent these pop-ups?

(in the past it was possible to install a tvOS beta profile to prevent this, but apparently Apple disabled this hack by making profiles device specific - we can no longer install a tvOS profile on an iPhone)

I have not heard of a way to do this, keen to hear if anyone has found one?