Wikipedia explains this character best
Venus de Milo was a female Ninja turtle who appeared in Ninja Turtles: The Next Mutation. In a 2007 interview director Kevin Munroe elaborated on the instructions Peter Laird gave to him for TMNT. Munroe admitted that among those rules was, "There’s absolutely no mention of Venus de Milo, the female Turtle. You can’t even joke about that with Peter. It’s just one of those things that he hates with a passion."
Is there any indication of why she was retconned out? He was a writer on the show, so was this something he'd always been against or did he change his mind?