Ny'alotha being N'Zoth's prison is player head-canon and not confirmed in any way, shape, or form to be a Titan-made prison for the Old God. Why when Il'gynoth is dying would he mention......if he's simply going to a Titan prison and not some Void-related afterlife akin to hell?"N'Zoth... I journey... to Ny'alotha..."
Xal'atath mentions in Stormheim...If she's comparing Black Empire/Old God/Void Lord structures to Titan structures, why would Ny'alotha be a Titan prison facility?"Do not be impressed by tall icons of the titans which stand here. The towers of sacrifice in Ny'alotha dwarf these pathetic temples."
The Puzzle Box of Yogg-Saron and Xal'atoh, Desecrated Image of Gorehowl (Y'Shaarj whispers) mention Ny'alotha as well, so why wouldn't they mention their own respective prisons in Ahn'Qiraj, Ulduar, and the Vault of Y'Shaarj instead of N'Zoth's?