There's been very little because it's very hard to do, to track people effectively over any long period of time. Some jurisdictions and organizations claim that they do follow up, but very often those claims aren't really very reliable.
The New South Wales police, many years ago now, did quite a systematic study of the people who were supported through their internal witness support program, police who had made disclosures about other police, and of how their welfare panned out over a period of time as compared to other groups within the police service. It certainly can be done, and it should be done, but apart from that study, there haven't been many substantial studies like it that I'm aware of.