FWIW, I think you should just depend on 1.1, eg specify something like
[compat]
julia = "1.1"
in your Project.toml. This is what I am doing for almost all of my packages (yes, I love Base.@kwdef too, and I am removing a lot of workarounds as the 1.1 compiler is so much smarter than 1.0).
Generally, it is my impression that features will not be backported, just critical bug fixes. Which is as it should be: with a lot of backports, one could hardly commit to keep a branch “stable”.
See the some discussion starting here.