*BSD "securelevel"

For example, the immutable files protection can be bypassed by mounting over the directory. It doesn't allow you to change the original file, but allows you to fool other applications at runtime and is thus of little use for, e.g., preventing root kit installation once you've already attained root.

AFAIK nobody has bothered to fix it on systems where it was an issue (NetBSD was immune to this particular attack). The fundamental issue is that even this course-grained capabilities system gives a false of security. Invariably someone will forget about some corner case, or some new feature is added which allows circumvention of the whole pile of policies.

Fine-grained capabilities systems (both system-level and process-level) are just too brittle, including the policies, the mechanisms, and the actual implementations.

Unix systems have only just recently reached a decent level of correctness and reliability with basic file permissions. Anybody who relies on more sophisticated schemes (or allows them in their kernel) is just begging to be rooted.