Every forecast can have different wells (project and/or company) included. The only way I know the wells included in a forecast is to pull the forecast output, and monthly and daily volumes. That is a lot of work for just wanting to know the wells included in a forecast. Hence get-forecast-wells API endpoint would help if it were available.
We do not currently have an endpoint to retrieve Forecast Wells directly. The closest workaround at this time is to use the get-forecast-outputs method and pull the Well Ids from the result set like you mentioned.
I have forwarded this request on to our Product Manager to determine if this is something that we can add to the API in the future.