The primary method was through scraping flightstats.com. About a month or two ago, FlightStats stopped displaying that data. The fallback method was using GTC credentials. However, those are generally not available for free, and as far as I know, KVS itself does not provide a means to obtain them (which is not to say that one cannot acquire their own from another source).

I believe there is still availability through the KVS mobile companion. However, that is limited to just the highest tier of "contributors", and suffers from not being able to look up flights before some time in the morning (I want to say 8am, but don't remember for sure). KVS has not answered questions about any forthcoming sources, nor why the mobile companion source isn't available in the desktop app.