It used to be the link icon, but that replaced the link icon where the share icon wouldn’t make sense (like on the formatting bar button for adding a link).
I just tested the below and it seems to work as intended:
api.replaceIcon('d-post-share', 'share-alt');
api.replaceIcon('d-topic-share', 'share-alt');
The first replaces the icon within posts, the second replaces the icon for the Share button at the bottom of a topic.