Alright guys I finally came to a conclussion. Battle Frontier is kill.

How am I so damn sure, especially given we have all those hints? Simple: the music. The whole OST was leaked, and a kind anon around here posted it and I was gladly listening to it. But I came to a realization: doesn't the Battle Frontier have exclusive music, too? And it happens that the Battle Resort theme is a remix of the Battle Frontier one and there is no Battle Frontier or any of its buildings' music anywhere (and we all know the Maison has a remixed Tower theme). If we truly were to had a Frontier there would be some themes alongside it.

Bottom line: There's no Battle Frontier coded into the game. Zero. Nil. All we have are some pretty references which can be seen as a huge "FUCK YOU" from Gamefreak. If it's going to be included as DLC in the future or not, that's not of my knowledge. But it's not a timed event, neither it requires to beat the Maison or anything.

That's it. We know all the postgame.