macOS High Sierra: Where is the Secure Empty Trash option?
This feature was removed in macOS 10.11 El Capitan. Per the Apple support document About the security content of OS X El Capitan v10.11, it was removed for security reasons.


  Finder
  Available for: Mac OS X v10.6.8 and later
  
  Impact: The "Secure Empty Trash" feature may not securely delete files placed in the Trash
  
  Description: An issue existed in guaranteeing secure deletion of Trash files on some systems, such as those with flash storage. This issue was addressed by removing the "Secure Empty Trash" option.
  
  CVE-ID
  CVE-2015-5901 : Apple


The CVE gives a small paragraph on this issue. From NVD - CVE-2015-5901:


  The Secure Empty Trash feature in Finder in Apple OS X before 10.11 improperly deletes Trash files, which might allow local users to obtain sensitive information by reading storage media, as demonstrated by reading a flash drive.