We recently upgraded from 1.7 to 2.3 and suddenly automatic index creation based on our templates no longer worked. We have time based indices, and we want them to be automatically created based on our templates, which defines strict mappings, so we have disabled dynamic mapping. After some debugging, it appears that this feature changed it's behavior as of the fix for issue #15381 so it is no longer possible to have indices automatically created when dynamic mapping is disabled. Is this really how it is supposed to work? Throwing an exception doesn't really seems like a fix to the issue in the first place, considering what this change breaks.
I could not find anything about this new behaviour in the reference documentation in the section about index auto creation: https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-index_.html#index-creation