Nullable's have been around for at least a decade and before .Net core existed? Are you thinking of the NotNull attribute argument checking instead(Likely the wrong term here)? Any change in a data type to an existing API or code will always be tough and a lot of work. Maybe an in32 to a in64 being the exception.

Library developers are looking at these and following them to make enhancements. Developers working on business apps get the automatic performance improvements of these changes and the library dev changes to improve performance and allocations. I personally quickly go over these so when I get into a performance issue I have a little bit to know where to go find more information.