El Capitan: will System Integrity Protection defence stop sandbox program from accessing System.log?
Will my program (running in the sandbox) be able to access the file /var/log/system.log with the 'System Integrity Protection' system running?


Yes, a sandboxed application will be able to access /var/log/system.log on OS X with System Integrity Protection (SIP) enabled.

/var is Not Protected

The /var folder is not protected by SIP. As of OS X 10.11, only the following locations are protected:


  Paths and applications protected by System Integrity Protection include:
  
  
  /System
  /usr
  /bin
  /sbin
  Apps that are pre-installed with OS X
  
  
  Paths and applications that third-party apps and installers can write to include:
  
  
  /Applications
  /Library
  /usr/local
  


See Apple's developer documentation on System Integrity Protection for more details.