There are a few major canon changes:
There was a big event, where all the earths were combined and no one remembers except the Arrow, Flash, and Supergirl teams. remember. Mon-el is never mentioned nor did he ever come.
Lauren and Iris are bashed so if you are a big fan of the two of them, I would not suggest you read this.
This does not take place in a specific season in any tv show involved.