Notes:

For those who don't participate Steam Client Beta (and doesn't get the joke): There has been attempts on adding support for more than 4 Xbox Controllers for some time.

More recently, SDL2 has defaulted to RawInput driver ( amongst other things ), which should allow them to support more than 4 Xbox/XInput controllers. Which, in the last four Beta Client updates, this experimental support has been popping more frequent than before, each update enables and disables it for further testing.