This pack gives trial mobs and skeleton horsemen bossbars, one per spawner and trap respectively. It can hold a maximum of 30 trial and 30 skeleton trap bossbars at a time. All types of trial spawners function perfectly, including custom ones from datapacks or spawn eggs.

The pack should work immediately upon applying. If trial mobs were already present before enabling the datapack, it is possible the bossbar doesn't appear yet, which just means they were too far away from the spawner to locate it. However, the next time that spawner spawns a mob, it should be marked and bossbars should then start functioning correctly.

As far as permanent changes to your world go, this datapack should only affect the following entities (no blocks or items):
 - All loaded persistent mobs will be checked to see if there are spawners near them, and once they have been checked they will be given the tag "checked.for.spawner" to prevent unnecessary double checking. These tags have no effect on the entity outside of telling the pack not to check them anymore.
 - All skeleton horsemen have a "marked.horseman" and "checked.for.trap" tag, and if they wander out of range they'll also get the "left.fight" tag. These tags also don't affect the entity, they just help the datapack keep track of them.
 - To be able to read the data of the trial spawners and skeleton horsemen, invisible markers are placed in their locations. These are used to store the positions of the spawners and to position the bossbars, and will be removed automatically if their spawner or skeleton trap gets destroyed. However, they will remain present after removing the datapack. If need be you can run the following commands to delete them:
/kill @e[type=marker,tag=trial.spawner]
/kill @e[type=marker,tag=skeleton.trap]

This is my first datapack so if you have any ideas for efficiency, bugs to report, or other suggestions, any feedback is greatly appreciated!

Ideas I'm considering:
 - Giving zombie sieges a bossbar. I am actively working on this but it is complicated due to zombie siege zombies being indistinguishable from regular zombies.
 - The ability to toggle whether bossbars show for trial spawners and skeleton traps.
Let me know if you agree with these!