Yep, as far as I can tell, it was composed specifically for the OST (which is one of the most innovative ones I've heard in recent times).
Usually the OST and OP/ED singles (and in this case as well as for some other anime, character songs) come out during the series run, especially if it's 26 episodes or longer.
Just go to Nipponsei's tracker, where a lot of the mp3s on Box are from.
https://nipponsei.minglong.org/Go to 'tracker' not 'releases'.