Hello,
mitigated risk in the estimation of risk after doing reducting actions.
It is a way to measure the efficiency of reduction risk actions.
For exemple, before actions risk is high probability and high severity.
Actions taken will reduce probability and other severity, at the end, risk is estimated probability low and severity medium.
It is a good pratice to estimate and keep trace of efficiency of actions, and it is fine to trade with client or aother teams about risks and how they are managed.
It is possible to add mitigatedProbability and MitigatedSeverity via custom screen but we lose the calculation of criticality (and, by the way, the caculation may be not the same, this is an issue for compare...).
And, when you display list of risks, with value without and with mitigation, you loose the coloring of fields and then lisibility (see above).