> (* Technically not all mutable objects live on the heap, because some never live at all, as they are optimized away so are never allocated in the first place.)

The compiler will often stack allocate mutable objects in Julia. This is not the same as "never existed in the first place", because the stack pointer gets incremented and underlying data layout is you load from it is the same as that of the mutable object you allocated.

Here is one example where that's very obviously what's happening: https://discourse.julialang.org/t/why-is-svector-faster-than... But it can happen now generally with mutable structures that can't practically just live in registers.