Hey folks,
we encountered a regression when updating to the latest 7.16.3 Java client, still making use of the RestHighLevelClient.
The main cause for the issue is that actually the GetIndexRequest.DEFAULT_INDICES_OPTIONS setup:
doesn't match the server defaults as of:
The main mismatches are:
- allowNoIndices:
server default=true
client default=false - expandToClosedIndices
server default=false
client default=true
In the past this hasn't caused any trouble as the client defaults were just always sent to the server. However since Deprecate ignore_throttled parameter by martijnvg · Pull Request #77479 · elastic/elasticsearch · GitHub they are not sent in case the current requests options equal the defaults of the GetIndexRequest.
Given that server and client defaults are not equal, the behaviour of the IndicesClient.exists thus changed when updating to 7.16.x.
I understand that the RestHighLevelClient is deprecated already though, we will work around for the time being by using a custom options config to fix the regression we experienced.
I still wonder if you may consider it worth evaluating whether the previous default behaviour can be restored 
I haven't checked the behaviour of the new client yet.
Best
Sebastian