At a minimum, we need some query that can return a list of certnames whose catalogs mention any one of a provided set of inputs. Where this should end up hasn't been determined yet.
Inputs would may also be returned under a new optional key in the catalog response. Whether or not we'll want/need to make that field opt-in is as yet undetermined, but our versioning policy does require clients to ignore unexpected new fields, so we're not obligated to make it opt-in, nor to increment the endpoint versions, and adding it to the catalog response means we wouldn't have to adjust sync or import/export.
An experimental [`catalog-input-contents`](./api/query/v4/catalog_input_contents.markdown) endpoint has been added. It allows querying for the most recent catalog inputs that PuppetDB has received for any nodes. ([PDB-4371](https://tickets.puppetlabs.com/browse/PDB-4371)