Altering accessibility at run-time is not possible if a security manager is configured (in Java) or the code doesn't have the right permission (https://docs.microsoft.com/fr-fr/dotnet/framework/reflection-and-codedom/security-considerations-for-reflection[in C#]).
So a new security-hotspot rule could be relevant in the future to detect when a loose permission is granted that allow, for instance, external code to change accessibility of fields of a trusted code and thus access to private information, however here:
* The rule raises when the accessibility is changed, so basically "the consequence" and not the root cause of the bug is highlighted.
* Reflection will be most of the times used to create dynamic types or to perform debugging.
So a code smell is more appropriate than a security rule.