When did “y’all” become improper?
It was never considered "proper" English; however, as cited above, it fills the need of the otherwise-absent second-person plural. It's very common vernacular in the South and some of the West of the United States. I'm not aware of it commonly being used outside the United States.
An argument for the superiority of "y'all" over "you guys," which fills the same grammatical niche in other parts of the US that "y'all" does in the South.
Economical: one word, one syllable.
Nonsexist.
Gracefully takes a possessive. "Is that you guys's new minivan?" vs. "Is that y'all's new Tesla roadster?"