add deprecation TODO comments for unused color vars

Author: shawnbot

src/support/variables/colors.scss

@@ -2,15 +2,18 @@
 // Color variables
 
 // State indicators.
+// TODO@14.0.0: remove $status-pending (unused)
 $status-pending:    desaturate($yellow-700, 15%) !default;
 
 // Repository type colors
 // Should be able to deprecate these in future
+// TODO@14.0.0: remove $repo-private-text, $repo-private-bg, and $repo-private-icon
 $repo-private-text: $black-fade-70 !default;
 $repo-private-bg:   $yellow-000 !default;
 $repo-private-icon: rgba($yellow-900, 0.5) !default;
 
 // Highlight used for things like search
+// TODO@14.0.0: remove $highlight-yellow (unused)
 $highlight-yellow: rgba(255, 247, 140, 0.5) !default;
 
 // Border colors