What does "S-ILS" signify in an FAA NOTAM?
The S is for the straight-in portion of that approach (this is explained in the Abbreviations section of the Terminal Procedures Publication (See here - it's in the Abbreviations section of "Legends and General Information" at the bottom).  In this case the decision altitudes, height above thresholds, and weather minimums have changed.  

Check out the approach plate and at the bottom you'll see S-ILS 15, S-LOC/DME 15, CIRCLING, etc.