To channel Victor: Another thing to look into is to introduce accessors for struct fields in category 1 and 2 so that the struct can be made private in the future.

The difference between category 1 and 2 is sadly not very clear cut. Anything defined in public headers could be used 3th-party code.

In this particular instance there is no documentation for the fields of the struct, which may indicate that the struct is private. However, this struct is used by code outside of the stdlib and there are currently no accessors that can replace this usage.