The people who are being suspended and expelled from the Labour Party are MPs and Councillors. So they have been in it since a lot longer ago than last September.
Ken Livingstone joined the Labour Party under Tony Blair. That is not a technicality. It is just a fact. He was able to do so only by special dispensation, a year before he would have been eligible to submit even an application that could have been rejected.
If anything, then, Jeremy Corbyn is the first Labour Leader to take anti-Semitism seriously.