Looks like it's undocumented, you can use it to fetch the latest user version on a build uploaded by butler.
Here's an example:
https://itch.io/api/1/x/wharf/latest?target=doborog/clonedroneinthedangerzone&ch...
Regarding the other error you're having, sounds like a encoding issue with the API you're using in Unity. Verify that the function doesn't take query parameters as separate object. Maybe the `?` is optional if query params are handled separately