That episode was an absolute banger, and yeah it was the conclusion of Justice League Unlimited but also served as the conclusion for Batman Beyond which never otherwise got a proper conclusion. And thus by extension it closed out that entire Bruce Timm DC animated universe that ran from 1992 with Batman to 2006 with Justice League Unlimited.

I should write a note about that episode actually. The Batman/Ace seen was one of the biggest gut punches in kids animation history.