Is union abstract type?

General Usage
2

This is the documented behavior of isabstracttype:

Determine whether type T was declared as an abstract type (i.e. using the abstract type syntax). Note that this is not the negation of isconcretetype(T). [emphasis added]

It sounds like you want !isconcretetype, e.g.

julia> !isconcretetype(IntOrString)
true
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