Its was datamined, together with other unused quest titles.
either way, i always considered Noctis sleeping during the 10 years to be Bahamuts trial, even before.
During another rewrite of the story, chapter 14 was supposed to be about Noctis in the crystal, with chapter 15 picking up when he wakes up. There was a whole chapter cut out that was gonna have Noct wandering the Forest of memorise i mentioned earlier, seeing memories of Luna and Regis. This chapter being cut is why Luna's cutscenes have jarring placements. It is also where Noct likely was to discover the truth of how his father died, as well as apparently featuring an secret memory locked away. It is also probably where Human Umbra and his ninja village were. This chapter being cut led to what little content there was to be turned into Platinum Demo. This is conjecture but the finale of XV kept getting scaled back. Episode Ardyn as a whole was 100% meant to be in the game. The whole DotF was basically the original concepts but expanded upon with new stuff.
Chapter 15, as Trial of Bahamut, likely would have had multiple endings. The one we know, where Noct fights Ardyn. Likely one where Ardyn wins, and then the Grand finale. Grand finale was likely too grand and got dialed back, leading to Luna's reduced role. Then it got reduced further, given Insomnia as a whole was only completed with Royal Edition.
I reread the datamine and old interviews, play old demos and even touch base upon the FNC now and then and every so often i come to a realization for why parts of the story were cut, expanded upon, etc.
They definitely invented new stuff for XV, but given Bahamut is the real antagonist, the real tragedy is they could only deliver us the bad end. We just never realized it. Guaranteed if we made different choices, it'd have affected Noct's outcome in the crystal. Given the grand finale was cut, it's like how omen did happen, in another timeline but Regis prevented it. Noct sees the main game ending as his destiny. Bahamut repeatedly shows Noct his destiny in the crystal. When he was thinking of his friends, Bahamut made him think of Ardyn.
I love XV but now i just can't play it fully knowing what we were supposed to get. Even the OG world map and maps hidden in Platinum Demo indicate further driving to all continents, the train starting from Altissia. The big chapter list with descriptors in th3 datamine is newer than the one with the Trials of Bahamut, Ramuh and Titan as chapter names. It seems "End of the Road" would have been literally Gralea, crashing the car. Umbra's time travel feature was likely meant to be the Crystal chapter in th3 forest of memories, allowing us to grind before choosing our destiny. trimming the ending led to there being no need so they gave us Umbra in Altissia.