Ain’t is a contraction or shortening of are not and am not.
Now it was actually proper English around 1778 in England and was used by the upper class as well as all classes.
But then around the 19th century popularized by Cockney speech and Charles Dickens people started using ain’t improperly for is not, have not and has not.
And the British got so upset that they banned the word altogether in proper English usage while words like “don’t” and “won’t” remained proper.
So, I ain’t is correct, they ain’t is correct, she ain’t is incorrect and they ain’t done it yet is also incorrect.
Do you think it’ll be OK if we use I ain’t in our everyday speech?