WordPress 4.6 has switched to system fonts. 🎉 You should deactivate this plugin if you're using WordPress 4.6 or higher.
A plugin for WordPress that replaces Open Sans with your device's standard user interface typeface.
There are other plugins that remove Open Sans, but they rely on the generic sans-serif fallback. This one adds a new stylesheet that specifies the system UI font for most major operating systems.
This is based on the approach we developed for WordPress.com and Calypso. For more background, see this Calypso PR: Replace Open Sans with system UI fonts.
Credit to this WordPress forums thread for the code that removes Open Sans. https://wordpress.org/support/topic/remove-open-sans-and-add-custom-fonts
Disclaimer: I never claimed to know what I was doing; if your site is hacked by pirates after installing this plugin, it's not my fault.