It's been on the ask list for Microsoft ever since they introduced the function, but
STRING_SPLIT providing the ordinal position of values in the string is something that I really wish they would do. I realise that it may well be a breaking change, due to people using
SELECT * far too commonly so if they don't want that happening they could introduce a second function that does return it.
Having it in the function is really important, as it's documented that is doesn't guarantee that the order of the values will be the same as they were in the string, and I commonly see people using
ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) or similar to "get the ordinal position"; that is not going to work at some point and could be very bad.
Of course, we all know that storing delimited data in an RDBMS is a big "no-no", but there are unfortunately often times we have to deal with it, either as incoming data and a historically poorly designed database, and making that dealing reliable and performant is really important.
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.