admin/plugins.php illustrates each plugin with its own icon. If an SVG icon is present, the SVG icon is preferred over PNG or other pixel formats.

However, if the plugin only offers an SVG icon, no icon is displayed at all.

Steps to reproduce:
1. Visit admin/plugins.php and confirm that mod_forum has an icon (Bonus step: Look at the used image; confirm that it contains SVG XML tags).
2. Delete or rename the icon mod/forum/pix/icon.png on your filesystem, leaving the SVG in place.
3. Visit admin/plugins.php again; confirm that only a spacer is shown instead of the mod_forum icon (empty image with icon pluginicon noicon CSS classes.

By the way, the activity module overview admin/modules.php respects the SVG icon as a standalone