The elasticsearch cluster running behind translatewiki.net should be upgraded to 6.8.23. Reasons are:
- the version (6.5.4) it is currently running is EOL
- WMF wikis are moving to 6.8 and then 7.10 which might introduce breaking changes to the client code (e.g. https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Translate/+/776221) in upcoming mediawiki versions.
Note that after upgrading to 6.8 all the indices should be re-indexed so that they pick-up the most recent default index settings/mappings.