In her final moments, Kara Zor-El saved the her universe and those of others by vanquishing the Anti-Matter. However, that was not the last of her, if former Brainiac Five had anything to say about it. In the 30th century, he subconsciously constructed a robotic version of Supergirl... vastly superior to the one she used to double as her alter ego... who carried on a romance with him for a time until she perished. Kara Zor-L remained alive and vibrant to inspire the romantic fancy of her most persistent suitor, Star-Spangled Kid, even through turbulent times such as in the notional dimension of Asgard where they briefly rekindled their unrequisite affections.
However, just as there was Zor-L as Power Girl defending her native Earth-Two in the timeline in which her adopted world survived the Crisis, there was another Zor-El in the form of the Linda Danvers clone. While no longer possessing superpowers due to exposure to Gold Kryptonite, perhaps when her "cousin" Kal-El discovered her existence (maybe from Kara's long-lost diary or some other source) he may have revitalized these abilities through artificial means to regain his Supergirl. This, along with the original Girl of Steel's 30th century adventures alongside Brainiac Five's Legion of Super-Heroes prior to her demise would make up for the missing Maiden of Might's presence on one Earth while she existed and aided the Justice Society on another.