Where have all the configurations gone?

Marlin configurations for specific machines are now maintained in their own repository at:

https://github.com/MarlinFirmware/Configurations/tree/bugfix-2.1.x

Configuration files for use with the nightly bugfix-2.1.x branch can be downloaded from:

https://github.com/MarlinFirmware/Configurations/archive/bugfix-2.1.x.zip