I received a warning notice written to my son dated October 2019 from a collection agency for past due court fees from May 2014. My son died in June 2016 in Marion County. Why in 2019 is he getting this?

What you have happing on is your son past due fees have gone to a collection agency. Company sell past due funds to collection agency for then to get some or all due. If you have file bankruptcy or died in your case then you owe nothing unless they talk you into paying something. At that point then they have got you on the hook for repayment of those funds.

Efficiency of our American government and electronic backlogging