Looks like you lucked out: Changelog
But it's still in use throughout EE code (for example):
$fts = ee()->addons->get_files('fieldtypes');
Defined in /system/expressionengine/libaries/Addons.php (line 50 v1.6.1)
...or perhaps the other one in /system/expressionengine/models/file_model.php
$files = ee()->file_model->get_files($dir['id'], $params);
But not being in the documentation or formally available to developers can mean that Ellis Labs can change it in the future and therefore render your module broken, so I appreciate you're following the tutorial (which obviously needs updating) I wouldn't use the above function in a commercial environment.
Unless, like the tutorial, they've just not updated the documentation to refer to the function...