This spreadsheet contains the settlement prices for the S&P 500 index options (SPX) and the Russell 2000 Index options (RUT). The data for SPX cover January 2006 through February 2021. The data for RUT cover January 2006 through December 2017. I stopped capturing RUT data because my stock charts provider stopped carrying RUT data due to a large price increase. Consequently, I stopped trading RUT and stopped tabulating the RUT data.
It is important to note how large a move may occur between the closing price for SPX and RUT on Thursday (the last time these positions may be closed), and the settlement price determined on Friday. The index settlement price is not equal to the opening or closing price on expiration Friday. These are examples of options with AM (morning) settlement.
Beware of allowing SPX or RUT options to enter expiration; you may be surprised. A one day standard deviation move on SPX is usually around $15-$18, and you will see that the difference between Thursday's close and the settlement price on Friday averages around $10 or less. But there are notable outliers, e.g., September 2008 and January 2016.
Always know the settlement characteristics of the options you trade, e.g., the weekly SPX options have PM settlement even though the SPX monthly options have AM settlement.