"In this paper we investigate strictly positive types which we define to be those types
which can be formed using 0, 1, +, ×, →, μ , ν with the restriction that types on
the left side of the arrow have to be closed with respect to type variables."

https://www.sciencedirect.com/science/article/pii/S0304397505003373?via%3Dihub

Finally, a good definition of what a strictly positive type is.