Hm, I'm confused, usually the whole point of storing an IP is in case the visitor uses the platform to do something illegal, like a death threat. Without the original IP law enforcement can't subpoena the ISP, etc. But also as someone else said, if you use a nonce, and I think you mean salt, then it can be cracked nearly instantaneously anyways due to the small space of IPv4 (~4 billion).
There is more to investigating then gathering admissible evividence.
For example, if you are talking with Alice and she says that she heard from Bob chat Charlie was in the office at a weird hour on the day in question investigators have gains nothing that they can admit as evidence [0]. However, there is nothing that anyone (defense or a third party) can do challenge this portion of the investigation other then keep it out of evidence, and the investigators are free to follow up with either Bob or Charlie to get something that would be admissible.
