Currently, the only two places where conditional defaults can be defined is $wgConditionalUserOptions in LocalSettings.php (for configuration made by a sysadmin) and ConditionalUserOptions in extension.json (for configuration coming from extensions). This means that all conditional defaults config in extensions need to be statically-defined.

GrowthExperiments needs to make conditional defaults config conditional, to ensure our own config (namely, GEHomepageNewAccountEnablePercentage, GEHelpPanelNewAccountEnableWithHomepage and GEMentorshipNewAccountEnablePercentage) is respected. Adding a hook to modify conditional user options would allow the Growth team to complete T383700, reducing the number of rows we create on signup.