In my experience you'd be hard pressed to find 'sth' used in the US (same for sb), most people would type it out to avoid ambiguity with 'south' and other words

In my experience you'd be hard pressed to find 'sth' used in the US (same for sb), most people would type it out to avoid ambiguity with 'south' and other words. In SMS and other short formats, phonetic spellings like 'sumthin' are more common. I've only ever seen South abbreviated as S, never as sth.