It is true that Bishop Fellay has redacted some of Archbishop Lefebvre's writings and/or sermons from SSPX websites. I cannot give any explanations for these actions, however.
I do not believe Archbishop Lefebvre died a sedevacantist; at least, I've seen no evidence that he was. During his life he clearly respected the sedevacantist opinion at times while at other times he was hostile to the opinion.