This is part of the 1.13 update (Pre-Release 1). You can read more about it here.

In addition to that stuff, the way SubServers.Host is launched on the inside has been rewritten. This means the way plugins are loaded and registered has also been rewritten as well. Getting your SubAPI plugins to load no longer requires a package.xml (just @SubPlugin the main classes), and parsing/loading from that is now done in a faster, more efficient way internally. It also means SubServers.Host no longer needs to create a temporary system directory to re-launch itself from.