Hi there. The shell just has a low drop rate from the Tier 5 boss chest, but it does have drop protection and is eventually guaranteed to drop. Regarding the ship, Bungie issued an update earlier this week that granted it to all players in their Collections, under Equipment > Ships > Season 8. Before then, the ship and shell drop rate protection were tied together, so if you got the ship first it reset your progress on the shell and vice versa. Now that everyone has the ship, you're guaranteed to get the shell from the final chest eventually.