Based on it's description, one would expect to be able to use 'is_leashed_to' in entity_sensor or environment_sensor to check for entities leashed to the calling entity. Currently, using this filter doesn't trigger the event of either sensor.

Repro steps:

1) Add entity_sensor to player entity to check for other entities leashed to it:
"minecraft:entity_sensor": {
        "sensor_range": 15,
        "relative_range":true,
        "minimum_count": 0,
        "maximum_count": 10,
        "event_filters":

{           "all_of": [             \{ "test": "is_leashed_to", "subject": "other", "value": true }

          ]
        },
        "event": "minecraft:gain_bad_omen"
      },
2) In game, leash any entity.

Expected: Player gains bad omen.
Result: Nothing happens.