The beta opt in panel prompts people to opt into beta.
It's been disabled for a few years now.
@ovasileva (on slack) said we could remove this code.
Config
$wgMFExperiments['betaoptin'] = array( "name" => "betaoptin", 'buckets' => [ 'control' => 0, 'A' => 1, ], 'enabled' => true, );