@stux @petko
So, let's say that I notice that someone has uploaded child exploitation content via their mstdn.social account. I report this to the police, and they begin an investigation. Part of that investigation is demanding that you provide them with the contents of that post, and any information you have about the poster.

If you've deleted the post, you cannot provide that information. By removing the post, you've deleted the evidence that a crime has even taken place, which could be seen (depending on local laws) as interfering with a police investigation.