How is this different/better than something like using a SHA256 of the input key?

Edit: Just looked it up... looks like this is basically what a uuid5 is, just a hash(salt+string)

This doesn't sound good at all. It's quite reasonable in many applications to want to send the same message twice: e.g "Customer A buys N units of Product X".

If you try to disambiguate those messages using, say, a timestamp or a unique transaction ID, you're back where you started: how do you avoid collisions of those fields? Better if you used a random UUIDv4 in the first place.