I'm not in a position where I can give you any empirical evidence because I do not collect it. My teams do see a decrease in bugs that reach production when they start to embrace immutability. But, the catch is that most of those teams are on a general journey of known good practices such as integration tests, static analysis, linting, and to some degree unit tests and reducing cyclomatic complexity. So I cannot in good faith give you any numbers because I do not run controlled experiments for that alone.