yes it can be used informally and formally and in academic papers even in newspapers and on television media reports. It is fully colloquial and not in a slang sense that many people mistakenly refer to as slang, bad or incorrect usage. This is a good description explaining that. SYNONYM STUDY 1, 2. Colloquial, conversational, informal refer to types of speech or to usages not on a formal level. Colloquial is often mistakenly used with a connotation of disapproval, as if it meant “vulgar” or “bad” or “incorrect” usage, whereas it is merely a familiar style used in speaking and writing. Conversational refers to a style used in the oral exchange of ideas, opinions, etc.: an easy conversational style. Informal means without formality, without strict attention to set forms, unceremonious: an informal manner of speaking; it describes the ordinary, everyday language of cultivated speakers.

It's more poetic than casual.

Yes, it is can be used in both formal and informal contexts. It can be used in writing (academic papers) as well. It is not slang but is very colloquial in use.

It can be used in both formal and informal language.