Explore the depths isn't a door, it's a cave, located on the top left side of the Depths. The Key is for a tombstone in that cave, which will open a passage
PoA assets are still to not be used without proper permission from Blue, and or the development team.
The PoA development team has plans to continue updating the mod, but is currently on hold.
I'm going to point something out.
Blue is not telling Fervis to not make mods.
He is telling him to make his mod, using his own scripts, and not PoA scripts. There is nothing stating that he cannot make mods at all.
He isn't being petty. He just doesn't want people using his assets, that he developed on his own time, being used without his permission. He never said the Eeveelutions were his, and Nintendo has nothing to do with this at all.