The closest filter to what the matrix smooth did in earlier versions of Surfer (with the default parameters) is one of the User Defined filters. Surfer 7 and earlier defaulted to setting the number of rows and columns on either side of the center to 2. This results in a 5x5 matrix. All the low-pass predefined filters are 3x3 (the low-pass filters will result in a smoothed grid like you want). Therefore, you can choose Linear Convolution Filters | User Defined Filters | Low-pass Filters | Moving Average and choose the Filter Size to be 5 Rows by 5 Cols. That should give a very comparable grid to the one matrix smoothed in the earlier version of Surfer 7 (you can't really tell the difference by looking at it).
If you want the newer Surfer grid to be as close as possible to the earlier Surfer grid, you could choose Linear Convolution Filters | User Defined Filters | General-User defined, choose the Filter Size to be 5 Rows by 5 Cols, and then modify the matrix a little. Make all the matrix values 1, except the center. Make the center value 2 (since in earlier versions of Surfer, the weight of the matrix center was set to 2). Then click OK. This grid may be slightly more accurate when compared to the earlier Surfer grid, but the difference is not significant and it is much easier to simply choose the Moving Average filter than to replace all the 0's with 1's when using the General-User defined filter.
Update October 22, 2018