unfortunately FlightStats pulled this tool from their website
Reason being they are funding their website and tools with advertisting.
It appears their paying advertising customers value more their new mobile app enhancements, and finally the great availability tool was now sacrificed.
To me the whole FlightStats website has now become useless and I am back to the level of the 90s with simple F/C/Y yes/no availability queries without fare bucket counters.

I found that the fare bucket counters have been the only reasonable indication how full a flight actually looks like and you could determine your options from there in terms which flights to pick, or determine chances for upgrades, etc.

I have no idea what other tool might still do the same job, I am not going to get a paid subscription for KVS or EF. In fact those tools might also become affected by the FlightStats tool removal.