https://joshuatreecounseling.us/2021/07/19/what-happens-if-i-tell-my-therapist-i-did-something-illegal/#:~:text=In%20Florida%2C%20duty%2Dto%2D,harm%20from%20a%20client%2Fpatient.

My layman's understanding is that if you say you committed murder 20 years ago, but your therapist doesn't believe you are actually a clear, present, and immediate danger to yourself or others, they don't actually have to report it. I find it hard to believe that there would be a situation where someone could admit to something like that and the therapist doesn't think they are at a reasonable likelihood to reoffend, but I guess the potential for the situation exists.

The link above is specific to Florida, but I'm sure that there are differences in law in different jurisdictions and probably even specifics at the federal level.

I am neither a lawyer nor a therapist, just a shitposter, so take all of this with a grain of salt.