Just checked and neither sucking or blowing. Also, the pool equipment has no mystery pipes or deadends... just the skimmer, drain, and return.
I think you are right that it is a placeholder pipe for some future work. I never thought much of it. The pool is from 2002 and I think I was hoping it was a return for a cleaner but since no suction.... I guess not without some extra work.
Thanks for the responses.