As far as I understand, wmf-config/InitialiseSettings.php should only contain settings with the following names:
- prefix wg – direct settings for MediaWiki core / extensions/ skins
- prefix wmg – used in some way in wmf-config/CommonSettings.php or somewhere else in the production config
- groupOverrides and groupOverrides2 – very special settings
Any other array key is probably a mistake (e.g. UseLegacyMediaStyles instead of wgUseLegacyMediaStyles), and it would be nice if CI could notify the user about it, instead of the setting (probably) silently having no effect.
(I expect this also goes for e.g. wmf-config/core-Namespaces.php and all the other settings that were split out of the giant IS.php in T308932 – everything that goes into composer buildConfigCache.)