Minecraft wiki entry for Nautilus_Shell, associated text: Nautiluses are animals that occupy deep oceans in the real world. However, they are absent in Minecraft and there are currently no plans to add nautilus mobs into the game.
Because Bedrock Edition allows for Nautilus shells to be held in the off-hand (since drowned hold them), they are the only item in that version that can be put in the off-hand slot without having a specific function.