The setting isn’t enabled by default because it isn’t needed for most sites. Since the text is parsed in the user’s web browser, enabling the setting adds some extra work to the browser. At the moment, the only way to test it out is to either join a site that it’s enabled on, or to setup your own Discourse site and see if it does what you’re looking for.
The setting is enabled on one busy site that I’m aware of, and seems to be working well for its users.