We have found 5 software records in our database eligible for .xlsb to .slk file format conversion.
A lot of spreadsheets programs still support Microsoft Symbolic Link format (.slk) and can export any of the common spreadsheets to it, including Microsoft Excel files (for example the binary XLSB workbooks).
Such xlsb to slk conversion (resaving really) should be easy enough with the standard File ► Save as.. function.
XLSB
Files with xlsb extension is used by Microsoft Excel spreadsheet editor distributed with the Microsoft Office. An xlsb file contains Excel workbook stored in a binary file format (BIFF12). The BIFF12 binary format is used for data that can not be stored in the Open XML text format.
SLK
An slk file extension is related to the Microsoft Symbolic Link, a data exchange file format (typically spreadsheets) used by various applications developed by Microsoft, but also supported by others.