Quite often these systems are "immutable" and the post is available to the admins if required. This was found in the Parler hack, for example.

The question is kind of moot because the police don't ask for evidence and don't pursue these cases. Can you dig up a case where the police made this argument?

Edit: oh the hilarity, here's the same discussion 22 years previously on the "decentralised" system USENET: https://www.wired.com/1998/03/canceling-the-evidence-on-usen...

I believe AOL lived to cancel again.