I am not sure about why, however I think it is safe to use it, if you check ?Base.@kwdef you will see it is well documented, what is often a clear signal it was intended to be used (specially considering the box saying it is available from Julia 1.1 on).
I find Base.@kwdef very useful and also think it should be exported.
Is there any specific reason why it could be removed in the future? Or is it just not documented because no one has done it yet ?