I agree that it would be better not to say anything negative if there is any liklihood that you may need to work for or with this company n the future, or if may want to work with other people in the same industry locally.
I once did give some negative feedback - not to HR, as the company didn't have any. I sent a letter, after I had gone. I never got any response (which pretty well sums up that company!)
But I was moving several hundred miles away and had already secured a new job and a reference.
Curious what your purpose was in sending a negative letter after you were gone and did you really expect a reply?
I left due to significant bullying and to practices in relation to clients which could result in significant losses to the business. There were 2 other employees who were also victims of the bullying - my purpose in sending the letter was partly a (not very strong !) hope that it might encourage the owners to address the issue and improve things for those individuals, but primarily to ensure that there was a paper trail that the bosses (who did not work in the same premises) were told about the issues, which would put the remaining employees in a much stronger position if they were sacked (the person who bullied me would threaten to sack people, and lied repeatedly about how others behaved - my letter meant it would not simply be his word against hers) or if they left and wanted to claim constructive dismissal.
I'm in the UK - there are laws which can entitle people who are sacked unfairly, or who leave as a result of bullying or other inappropriate behaviour by employers to claim compensation.
So by sending it (and letting my colleagues know that I had written it, although I did not give them a copy) I was doing what I could to help them deal with the bullying etc.
I was not surprised not to get an response but again, as I worked in a branch office so the majority of the business owners never saw what it was like there, I thought there was a possibility that they didn't know, but would care if they did know. I think it would have been appropriate for them to have at the very least acknowledged the letter, even if they did not want to respond in any detail.
but I only felt able to do it because I was going far enough away that I would not be working with them, or with any one likely to be working regularly with them (and because I had already told me new employers why I had left the old job!)