That is because prompts were introduced.
If a prompt is supplied, it already has the input and model references.
Before input and model being optional, when supplied a prompt it would still require these fields, which was why now they are optional.
It’s not like we don’t need to supply any parameters, but that is another challenge, perhaps we now need to find a better arrangement for improving the docs on what you pointed.
For reference: Enhanced Prompt Management - #56 by kwhinnery