As far as I know she's pretty important to the place's lore Thousands year old alpha predator that's practically mythology at this point. I woulda thought she'd be on the site with all the other Shurima peeps but I guess not :/
DwarvenGiant2 votesThis response from Jellbug in the recent Shurima Story Q&A might just answer your question:
While Rek'Sai does burrow through the Shuriman desert, her identity is inexorably tied to the Void. We're definitely looking into telling her story in the future.
Trolosaurus2 votesThis response from Jellbug in the recent Shurima Story Q&A might just answer your question:
tyvm
Nahui1 votesShe's not considered from Shurima, she's Voidborn and belongs with the rest of the Voidborn.
Trolosaurus4 votesShe's not considered from Shurima, she's Voidborn and belongs with the rest of the Voidborn.
But she dwells in Shurima Taric is from Demacia but he's on the Targon website because of how he's tied to it. In my opinion so should Rek
Skelenth1 votesThe Shuriman story revolves around Xeraths betrayal and the events following Azirs resurrection, and as you can see, they don't have that much to do with Rek'Sai.
Arakadia2 votesIt COULD have much to do with Rek'Sai. Rito decided to push her out I guess.
EDIT: For example it could be that when Xerath destroyed the city, a portal from the Void appeared which let loose Rek' Sai.
Skelenth1 votesRek'Sai most likely spawned from the icathian void portal like Cho'Gath and Kog'Maw. Also it would feel overdone if every champion related to Shurima would come up in a story about the Ascended. Zilean, Vel'Koz, Ezreal, Malzahar, Kassadin are all champions who at least at one point, were in Shurima or have really tight bonds with the desert, but they aren't part of the Ascended storyline. At least not yet.
Digielf1 votesreksai is clearly a void champion