Do zamknięcia, da się.
Jakby ktoś chciał
command /bb [<text>]:
trigger:
set {_bar} to new bossbar
set title of bossbar {_bar} to "%coloured arg-1%"
set color of bossbar {_bar} to BLUE
set style of bossbar {_bar} to SEGMENTED_20
add player to bossbar {_bar}
wait 3 seconds
remove player from bossbar {_bar}