Y'all is not slang, it's part of our dialect.
Yes, I've adopted it to my personal version of Standard American English. I spent most of my childhood in the Deep South, though I always considered myself a Yankee (as Southerners define it). But English needs a distinct plural form of "you" and since that old word no longer has that unambiguous function, I like the organic form of the Southern construction. It seems natural to me anyway. Less forced than that other innovation of mine, using "they/them" as generic intersex pronouns in place of privileging "he, him" which I know was an artificial "reform" imposed by scholars with a frankly sexist agenda.
The contracted form y'all certainly conveys a sense of informality, sometimes I stretch it to "you all." But we need it in modern English and I won't do without it!