You are right, this seems to only exist in .net core 2.0 and forward. https://docs.microsoft.com/en-us/dotnet/api/system.string.split?view=netcore-2.0#System_String_Split_System_String_System_StringSplitOptions_

are your solution with .net core?
.net framework 4.x doesn't contains String.Split(string, ...)

Aaah, nice one. I attempted something similar but I had to add an extra filtering step to get rid of empty entries. Now I understand how I could have done it better!