Originally Posted by
Myskin
Yep, that's my point - the "Waid being forbidden from writing Superman" incident I mentioned before happened after Birthright, not before it. Sorry for not being clearer.
As for the canonicity of Birthright, well, that's another problem. Over the years I have heard all kinds of stories about it - that it was supposed to be a "Ultimate Superman" thing or a possible pitch for a Superman movie. The original plot for Birthright (with Brainiac as one of the main villains) diverged even more from Byrne continuity, so there may be some truth behind it.
I think that at least at one point in earlier stages it was supposed to be canon (I remember reading an interview where Waid was asked if after Birthright he was supposed to write Superman and he half-jokingly answered: "Well, I better be!" or something like that). At one point there was a short story, "Young Luthor in Smallville", in Superman/Batman Secret Files (2003), written by Waid, which is clearly in continuity with Birthright (it also partially retcons some plot points of Birthright!); but at that point it was already clear that Birthright had become canon (Birthright Jor-El and Lara appear on the first page of Superman/Batman 1).
My personal guess is that they simply hadn't a precise plan in mind and they made things up along the way. The years which followed were probably the most chaotic period in DC's history as far as I know (I remember this utterly bizarre mini by Giffen, Ambush Bug Year None, full of incomprehensible and almost sinister in-jokes about the state of DC in 2008-2009).