Prior to 2.4, validation was not performed upon creation of an index template. Rather, the index template was validated upon index creation after all matching templates were fully merged.

Each index template must now be independently valid prior to merging. This means, for example, that you can no longer reference analyzers, filters, or tokenizers from a shared template.

See elastic/elasticsearch#21105 and elastic/elasticsearch#20479 for discussion concerning the new behavior.