A v8.6 user on the forums reported experiencing OOMEs and when they analysed the heap dump they found that a high fraction of their 3GiB heap was used by the fielddata cache. GET /_nodes/_all/stats/breaker?filter_path=nodes.*.breakers.fielddata agrees:

{
  "nodes": {
    "K6V95L0pR36L-_99LIapdw": {
      "breakers": {
        "fielddata": {
          "limit_size_in_bytes": 1288490188,
          "limit_size": "1.1gb",
          "estimated_size_in_bytes": 2766456144,
          "estimated_size": "2.5gb",
          "overhead": 1.03,
          "tripped": 0
        }
      }
    }
  }
}

They have worked around this problem by setting indices.fielddata.cache.size: 1gb but I think it's a bug for the fielddata cache to grow without bounds by default like this.