Direct answer

The async entity tracker works on which entities are tracked for clients, not their behavior trees. The Leaf 1.21.1 reference exposes enabled, compat-mode, max-threads and keepalive under async.async-entity-tracker.

How to check it

  1. Record entity density, number of viewers and client-side symptoms such as NPCs or entities disappearing.
  2. Confirm the exact generated config and test with the plugin stack used in production.
  3. Reproduce busy lobby or farm conditions and check for tracker worker errors or packets arriving unexpectedly.

What to change

Use the published setting only when your branch contains it:

async:
  async-entity-tracker:
    enabled: true

Test NPC visibility, disguised/player-type entities and packet-heavy plugins before rolling out; if NPCs disappear, review compat-mode rather than raising thread counts immediately.

Compatibility and safety

On the published Leaf 1.21.1 config, max-threads=0 selects automatic sizing. Do not take the historical thread fraction or default enabled state as a claim about current Leaf releases.

Software and version notes

The cited Leaf global config page explicitly describes the 1.21.1 branch; newer Leaf builds may have different keys, defaults or feature support. Verify config/leaf-global.yml generated by your actual server.

Verify the result

Observe whether visibility stays correct while the busy area is loaded. Compare entity tracking cost, client behavior and CPU contention before enabling it for all players.

References