I don't think it was ever actually confirmed, but I think the scenes featuring the LW in KH3 seem to confirm that it isn't canon. Mainly because SDG don't seem to recognize the LW itself, and Sora acts like it's the first time he's heard of it when Namine explains what it is to him in Re:Mind. Of course, Sora also didn't recognize the black coat in CoM after the fight with Xemnas in KH1 FM, so it could be Sora is just really bad at remembering these things. As for the LW's destruction, I think Nomura did say that it helped to merge Terra's components back into one.