My country has a law which says that rape and murder will not be punished if the perpetrator has managed to evade getting caught for 20 years. Recently, a case which involved both offenses passed the 20 years mark.

So I can totally imagine somebody bragging about crimes he has committed 20+ years ago, especially on the internet where reasonable anonymity can be achieved quite easily.

It would still be unwise. They can pass a law prohibiting people from hating you for what you did 20 years ago, but they can't enforce it.

If they pass a law to avoid punishment, they can later pass a law to allow punishment.

Same here, I imagined some remorseful ex-killer confessing some dark deed from way back in their youth. I guess it's related, but not quite the same thing. Good story though.