BossBars can now be enabled in settings.yml, at
Options.Use-BossBars (enabled by default, disabled in legacy versions)

This feature is only for 1.13+ servers, which has native BossBar support.

• Objective BossBar:
  • When a player steps on a objective, a bossbar will be shown that can be changed at
    Messages.Objectives.Boss-Bar-Format
    • %team% placeholder returns the amount of players of the team that owns this objective at the moment.
      • This changes accoding to who the capture is ticking to. For example, if the objective is switching to RED, %team% will return the amount of red players on the objective.
    • %enemy% placeholder returns all players from other teams that are touching the objective, adding players from Teams B (and C & D if available).
      
    • %name% placeholder returns the objective name.
  • You can also change Messages.Objectives.No-Owner, this is a failsafe if a team is disbanded (all players of a team leaves and the point is ticking)
  • The BossBar is dynamic, so it updates the "health" according to the captured percentage of the objective.
• Objectives now tick the player amount, so it can now be accessed via the API via Game -> Objective List -> GamePoint#isOnObjective. Check the wiki for more info.

CTF should also receive BossBars in a later update. A common request is the possibility to change the objective size, this is also coming in a later update. I'm basically spliting this update in two parts, this being Part 1.