Looks like forward slip to landing has been in the Private Pilot Practical Test Standards [1] since at least Nov 2011
[1][PDF] https://www.faa.gov/training_testing/testing/test_standards/...
[1][PDF] https://www.faa.gov/training_testing/testing/test_standards/...
https://www.faa.gov/documentLibrary/media/Advisory_Circular/...