This plugin may be re-activated on demand, as since SonarQube 4.5, RCI Metric has been re-activated thru the Issues Density Plugin
Computes a new metric named violation density. This is kind of the "opposite" of the rule compliance metric. The rule compliance metric's formula is: max(0, 100 - weighted_violations / Lines of code * 100). The violation density metric's formula is: weighted_violations / Lines of code * 100. The main advantage is that bad projects can always be compared because the formula allow metric values to exceed 100% while rule compliance can never be under 0%.
The new metric key is:
There is a new widget to replace the Rules Compliance one.