hell, I'd argue that it should be fine for formal writing as well. Ideally, the gap between written and spoken language is as narrow as possible, since ideally a written text communicates with the reader directly, with as little hurdles to parsing the content as possible. Having to maintain a secondary vocabulary is exhausting and creates barriers. There is a limit to how permissive I am with this personally though, generally I'd only be happy with changes that are semantically near identical, or introduce words which represent genuinely novel concepts rather than slang that re-brands existing concepts.