Of course, slang has often become accepted English, in phrase if not individual word. Sometime it goes wrong, so that "Donkey's Years" (a long time) is derived from the Cockney rhyming slang "Donkey's Ears" just meaning Years.
Some rhyming slang has at least become respectable, "berk" meaning a foolish or slightly objectionable person, comes from "Berkeley Hunt" or "Berkshire Hunt". "Tea Leaf" (thief) is also widespread.
With regards poetry, it could be argued that the entire language has followed the lead of Shakespeare-era poets in dropping the pronunciation of the "e" in such words as "learned" and "loved", which would be done in poetry and verse-plays to make a line fit the metrical scheme.
Yes, I am aware that the last "E" is still heard in "learned friend" in Court, but you know what I mean.