Hi @protoz,
That endpoint was added in Mattermost 3.5 so you'll have to upgrade your instance if you're going to use it. If you're not wanting to upgrade right now, may I ask what you're trying to do with this? I can try to point you to the right APIs for it
As for the API reference, they're for our latest released version (currently Mattermost 3.5.1). We're planning to keep documentation for each version starting from 3.5, so there's no documentation for 3.4 or earlier.