As noted in the extension.json documentation and demonstrated at T283464#7106236, the array_merge_recursive merge strategy is not intuitive and not recommended. In fact, it does not appear to be used in any of the extensions or skins that codesearch is aware of.
The array_replace_recursive merge strategy is likely more useful, although also seldom used.
To prevent confusion and accidental use of array_merge_recursive when the behavior of array_replace_recursive is likely desired, the array_merge_recursive merge strategy should be removed.