android 5.1.x security settings missing after upgrade
Based on on description, I think you might be referring to Samsung's Smart Manger. Smart manager gives an overview of the device's RAM, Battery, Clean up as well as Security (contains sn anti-malware engine).


  Smart Manager's functions are to manage storage and RAM, scan and
  optimize data usage to preserve battery levels, and protect from
  security threats, close all apps running in the background that
  significantly drain the battery, uninstall unneeded applications, and
  run malware scans.


See below:



It is most likely the OS update didn't have Smart Manager. However the good thing is Smart Manager has been renamed Device Maintenance and is available from Google Playstore as long the device is compatible