Elasticsearch 6.7.0 status Yellow in Magento 2.3.1

It's not unusual for a small system. I don't know anything about Magento, I'm afraid, but as a general principle you'll have to be aware of a couple of issues:

  • you only have the resources of a single server available to do all the work. Perhaps this is enough, perhaps it's not - it depends on how heavily you're using it.

  • you only have one Elasticsearch node. If it breaks (e.g. the disk becomes corrupt) then you will lose data.