Today when an index is created from shrinking or splitting an existing
index, the target index inherits almost none of the source index
settings. This is surprising and a hassle for operators managing such
indices. Given this is the default behavior, we can not simply change
it. Instead, we start by introducing the ability to copy settings. This
flag can be set on the REST API or on the transport layer and it has the
behavior that it copies all settings from the source except non-copyable
settings (a property of a setting introduced in this
change). Additionally, settings on the request will always override.
This change is the first step in our adventure:
- this flag is added here in 7.0.0 and immediately deprecated
- this flag will be backported to 6.4.0 and remain deprecated
- then, we will remove the ability to set this flag to false in 7.0.0
- finally, in 8.0.0 we will remove this flag and the only behavior will
be for settings to be copied
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