I thought maybe it was resolved by this PR:

#8865

But I am still seeing this message printed every few seconds (Kibana 5.4.3 talking to ES 5.6.3). it isn't possible to upgrade since the kibana plugin I'm using is only written against 5.4.3.

{"type":"log","@timestamp":"2017-10-18T04:52:25Z","tags":["warning"],"pid":1,"kibanaVersion":"5.4.3","nodes":[{"version":"5.6.3","http":{"publish_address":"10.43.188.45:18923"},"ip":"10.43.188.45"},{"version":"5.6.3","http":{"publish_address":"172.25.69.187:18281"},"ip":"172.25.69.187"},{"version":"5.6.3","http":{"publish_address":"10.232.131.86:18308"},"ip":"10.232.131.86"}],"message":"You're running Kibana 5.4.3 with some different versions of Elasticsearch. Update Kibana or Elasticsearch to the same version to prevent compatibility issues: v5.6.3 @ 10.43.188.45:18923 (10.43.188.45), v5.6.3 @ 172.25.69.187:18281 (172.25.69.187), v5.6.3 @ 10.232.131.86:18308 (10.232.131.86)"}

{"type":"log","@timestamp":"2017-10-18T04:52:27Z","tags":["warning"],"pid":1,"kibanaVersion":"5.4.3","nodes":[{"version":"5.6.3","http":{"publish_address":"172.25.69.187:18281"},"ip":"172.25.69.187"},{"version":"5.6.3","http":{"publish_address":"10.43.188.45:18923"},"ip":"10.43.188.45"},{"version":"5.6.3","http":{"publish_address":"10.232.131.86:18308"},"ip":"10.232.131.86"}],"message":"You're running Kibana 5.4.3 with some different versions of Elasticsearch. Update Kibana or Elasticsearch to the same version to prevent compatibility issues: v5.6.3 @ 172.25.69.187:18281 (172.25.69.187), v5.6.3 @ 10.43.188.45:18923 (10.43.188.45), v5.6.3 @ 10.232.131.86:18308 (10.232.131.86)"}

{"type":"log","@timestamp":"2017-10-18T04:52:32Z","tags":["warning"],"pid":1,"kibanaVersion":"5.4.3","nodes":[{"version":"5.6.3","http":{"publish_address":"10.43.188.45:18923"},"ip":"10.43.188.45"},{"version":"5.6.3","http":{"publish_address":"172.25.69.187:18281"},"ip":"172.25.69.187"},{"version":"5.6.3","http":{"publish_address":"10.232.131.86:18308"},"ip":"10.232.131.86"}],"message":"You're running Kibana 5.4.3 with some different versions of Elasticsearch. Update Kibana or Elasticsearch to the same version to prevent compatibility issues: v5.6.3 @ 10.43.188.45:18923 (10.43.188.45), v5.6.3 @ 172.25.69.187:18281 (172.25.69.187), v5.6.3 @ 10.232.131.86:18308 (10.232.131.86)"}