I would think a lawsuit would be a much bigger concern than criminal charges over the missing pool fence...
The fence law being changed wouldn't really do anything to protect him from liability over the man drowning in his pool. Criminally, you wouldn't be charged after a change in the law after it was abolished, but no effect on a civil case.
I doubt the punishment would have been that harsh for the missing fence, anyway. The fact that someone drowned in the pool wouldn't lead to more severe punishment as far as criminal court goes. I suppose there might be a small chance they could go after him for manslaughter, but that seems unlikely to me. The person who drowned was an adult who was trespassing, after all. I'm not a lawyer, though. I'm just going by what I've seen in articles and such over the years. But changing the fence law wouldn't really have an impact on a possible involuntary manslaughter charge anyway. The question in an involuntary manslaughter case is the degree of negligence involved, I believe. A fence law being changed after the death wouldn't really change that.
Of course, even if I'm right, Larry could have falsely believed it was key. I wouldn't call it a plot hole. Actually, that would just make it funnier, if everything he did to get the law changed was for nothing. Maybe that's how season 11 ends. I haven't quite finished it.
I know I'm overthinking. Season 11 just got me a little curious about how the real world law would work in a situation like this.