I think those can all be const as you only assign each reference once. You might think that value cannot be const as you write to value.value, but the variable value is, effectively, a pointer to a structure in memory, and that pointer is not changed when changing the contents of the object, so writing to value.value does not change the variable itself.
In theory using const may allow the interpreter/compiler to generate more efficient code, but whether that is actually the case in the node-red environment I don't know. It does stop one accidentally overwriting a variable that should not be overwritten.
I believe in practise it's more about the latter. I assume most function nodes are quite short so it might not be very likely to catch any errors. But I tend to also use const when possible out of the habit of it being the recommended practise for bigger code bases which I encounter daily at work.