Are integers immutable and are they acceptable as keys in

General Usage
2

Yes and yes (assuming you mean some concrete type like Int or Int64 or Int32). In fact, Ints are “bits” types, which is an even stricter subset of immutability:

help?> isbitstype
search: isbitstype

  isbitstype(T)

  Return true if type T is a "plain data" type, meaning it is immutable and contains no references to other values,
  only primitive types and other isbitstype types. 

julia> isbitstype(Int)
true
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