Ilm-history system index does not start with a dot


Here are the typical system indices found when using the Elastic Stack 7.6 (ES and Kibana are relevant here):

GET _cat/indices?h=i&s=i

I am wondering why ilm-history is not declared as a system index (which by convention start with a dot). For me, ilm-history feels very similar to .watcher-history for example. Am I missing something?