Another issue that also arises, if someone posts something explicit or hurtful towards someone, the person it was hurtful towards sees it but it is then deleted and no screenshot proof is provided, then no real punishment can be given as it becomes an argument of person 1 vs person 2.
Also, with not deleting posts, it deletes records of things that have happened, for example, someone may be warned for their post & then deletes it, this will delete their warning points & will also make it so that there is nothing on the forums saying that they ever did anything wrong and repeat offenders would look like new offenders.