This doesn't appear to be used anywhere, so let's deprecate it. Going forward we want to move away from PHP string concatenation inside Skin.php and embrace templates.
Skins that don't wish to use Mustache should do this kind of skin concatenation themselves.
https://codesearch.wmcloud.org/search/?q=getIndicatorsHTML&i=nope&files=&repos=
Note: The line in WikimediaApiPortal Skin is actually commented out so no change there is needed.