ThreadPool.java creates a background thread to updated an estimated clock time at an interval that is controlled by a configuration setting called estimated_time_interval.
Due to the way the component's settings are parsed this must be declared in configuration with the name threadpool.estimated_time_interval - BUT, earlier in the constructor of this class the call here throws an error if this name is defined in the configuration:

Map<String, Settings> groupSettings = settings.getGroups(THREADPOOL_GROUP);

The reason is that the getGroups call checks that all settings prefixed with threadpool. must be "grouped" i.e. qualified with the name of a threadpool e.g. threadpool.search.size.
Consequently we cannot possibly change the default setting of this interval and the need to do so has emerged as part of controlling search timeouts (see "Setting timer accuracy" section of #9156). A PR for this timeout issue includes a fix that changes the setting name to threadpool.search.estimated_time_interval.