We don't get to decide what gets treated as a typo (and accepted with a simple warning) and what gets rejected. It's an algorithm that determines it and we can't change how it works. If you got your answer rejected because of a typo, take note of it and write it correctly the next time.
It's not slang - it's used interchangeably with "help", both in speech and in writing, in American English and has been for generations. It may have started as a colloquialism in England in the 1600s, where it first shows up, but is pretty firmly part of everyday speech.
Maybe you meant to say it's an idiom?