how to access last index of array from split function inside awk?
If your problem is exactly as the example in your question, take the answer from @muzido, $NF will give you the last field.
If you just want to know the last element of an array by split():
split() function will return you how many elements it has just "splitted", test with your code: awk '{print split($1,A,".")}' file you will see the number. Then you can just use it by:
awk '{n=split($1,A,"."); print A[n]}' file # n is the length of array A