@Lostspark I know this is a Chabad thread, but maybe you can speak in terms familiar to those outside of Chabad? Like what do you mean “the Rebbe’s nisiyus”? What is the “seforim story of heh teiveis”. Who thought the previous Rebbe (and by this I assume you mean the Lubavitcher Rebbe before Rav MM Schneerson ZT”L. Because technically the previous Lubavitcher Rebbe is the one who passed on 25 years ago, since there’s no current Rebbe.).
But if as you imply, there were people saying that a dead man is still alive, then yes. That would be considered either avodah zara, or stupidity to literally any frum Yid.