LinkedList, along with a few other standard library types including HashMap, Box, or Vec, uses the unstable (and unsafe) attribute #[may_dangle] on their Drop implementation to get a more lenient drop-check behavior. With that attribute being used, it’s necessary to add a PhantomData that allows the dropcheck to know that dropping LinkedList<T> will still drop a value of type T, whilst – in contrast with the situation if the #[may_dangle] wasn’t used at all – it will also know that dropping a LinkedList<T> will not result in arbitrary access to values of type T.
This has the effect that e.g.
dropping a LinkedList<&'a u8> will be allowed even after the pointed-to u8 has already gone out of scope – Rust Playground
this would not be allowed if LinkedList didn’t use #[may_dangle] attribute
dropping a LinkedList<Foo<'a>> will not be allowed for a type struct Foo<'a>(&'a u8) if the struct Foo has a custom Drop implementation (which is necessary for soundness since this drop implementation could dereference the &'a u8 reference) – Rust Playground
without the PhantomData in LinkedList, this second point would not be ensured (in the presence of #[may_dangle]) and such a dropping action would be unsoundly allowed