I feel the reason why the Monster Hunter stage isn't listed on the official Smash Ultimate website is possibly one of the following reasons:
the stage isn't finished yet and is going to be redesigned into more than just "flat walkoffs with dragon"
it isn't a proper stage, just part of an event match, like the trophy stages in Melee (my best bet personally)
a new boss battles mode (not a story mode, they would've revealed that at E3 if it were in, the game would probably have less content because of it and the dragon would be in a generic location as opposed to a Monster Hunter location. SSE used all generic locations except Skyworld, Halberd and Ruined Zoo, but Halberd was the only one with a direct connection)
it's the sole unlockable stage