$wgConf->settings = [ … 'xSomething' => [ 'default' => false, 'testwiki' => true, ] ];
What should happen on wikis that are not testwiki? It creates $xSomething with the value false, which seems expected.
$wgConf->settings = [ … 'xSomething' => [ 'testwiki' => true, ] ];
What about this one? As implicit null? Or not exist at all? Not exporting a variable in that case would allow site administrators to conditionally create a variable (for variables that are custom to the site administration and not part of the software). It would also allow conditionally overriding the software default (which is separate from the wikifarm default), and might be used as a way to avoid repeating the default in the config repository.
In any event, there isn't an obvious outcome here. If the above theoretical use cases are real, we might want to support it. Otherwise, it might be best to disallow it entirely, or to warn for it.
Lastly, the reason this task exists:
$wgConf->settings = [ … 'xSomething' => [ 'default' => null, 'testwiki' => true, ] ];
Surely this should create $xSomething with the value null? However, it appears this currently results in the variable not being created at all - which led to the outage referenced in T225212 and documented.
Perhaps there was a reason for behaviour existing, and we may need to support it still. Or perhaps it is an accidental side-effect of isset() being used somewhere instead of array_key_exists.