I’d figure, it mainly is not advised, because it is intended just to use it for built-in post types, just because you can use for custom post types it does not change anything on that intention.
For example, if you use it and there are changes in WordPress that make that arguments use strictly available for built-in post types, then you are in trouble.
And, no, its not going to be removed soon – if ever, it likely will die with WordPress.