With 1.13 Vanilla MC added functionality for custom boss bars, usable via the /bossbar command. The Bukkit BossBar API does not interface with this functionality yet but uses another method of creating boss bars with fewer features.

The new boss bars use a namespace key based registry (e.g. youplugin:yourbossbar) in the server. It therefore would make them persistent and easily retrievable as opposed to the transient current implementation that requires the plugin author to keep a reference of their boss bars.

Additionally the custom boss bars allow using integer values and max values in order to indicate progress.