This popup window will not appear when opening a few apps from reputable developers and a few other cases are detailed in the Apple SKEL page.https://developer.apple.com/library/content/technotes/tn2459/_index.html#//apple_ref/doc/uid/DTS40017658
At first glance, macOS users will be pleased when Apple attempts to improve security on the OS with SKEL. But Patrick Wardle, an Apple security researcher and head of security research at Synack, doesn't think so.
Wardle said that Apple 's SKEL system "only prevents efforts of good intentions" (third party macOS developers such as security software designers).
He thought that 'due to the error in doing so, those who have bad intentions may not be affected' by the protection method of SKEL.
Disclosure of details on how to pass SKEL
To prove what they said, Wardle pointed out how an attacker could bypass the SKEL protection on macOS High Sierra via the Synack website https://www.synack.com/2017/09/08/high- sierras-secure-kernel-extension-loading-is-broken / and my personal blog. https://objective-see.com/blog/blog_0x21.html
'Unfortunately, when such security features are offered, even with the most attention, it only makes it difficult for third-party developers and users, while not affecting bad guys ( those who are not 'playing by law.' Wardle said. 'SKEL of High Sierra is a perfect example.'
Passing SKEL is only part of the study of Wardle's techniques for overcoming KEXT downloads starting last year and was introduced in detail at the DEF CON 2016 security conference.
MacOS High Sierra will be released on September 25.