We have a job that runs https://pitest.org/, analyse the report and tweak the codebase as per the results. Not sure it's ever found a bug that's likely to happen in prod but definitely gives us a confidence boost
We use this as well. It caught some lurking bugs when we first turned it on. Since then it catches things before merging, so it is harder to keep metrics. It did just point out a bug in a PR of mine the other day so it’s at least doing something :)