Noticed that IFS requires at least Excel 2019, and I only have 2016. A ods test file can be opened by LibreOffice Calc, nevertheless.
Committed above implementation with your registration line
Trying to open the ods test file with Spready (simple spreadsheet demo for fps), however, fails. Peeking into the ods file I get the impression that the function should be named 'COM.MICROSOFT.IFS' rather than 'IFS' -- this way the ods test file is readable by Spready. However, very probably Excel might reject a formula with that name...
Difficult
If you have a new-enough Excel version in which IFS is implemented, submit it here so that I can look at how the formula is named.