The only theme we really use from there is GitHub - silverstripe-themes/silverstripe-simple: The "Simple" theme for SilverStripe by Sara (Innovaif). Winner of the 2012 theme competition.. We’re already hosting other themes on github.com/silverstripe (see SilverStripe Ltd. · GitHub). So I’m proposing we move the simple theme there as well. This shouldn’t affect any composer installs, since the identifier in packagist stays the same.
For the other themes, they’re no longer maintained. I propose moving them to SilverStripe Archive · GitHub