Are these Sufis who claim that their sheikh has not died and will intercede for them on the Day of Resurrection considered to have committed major shirk, and how should they be treated?

Supplicating the dead, the absent, or the living concerning matters that none but Allah can accomplish is major shirk (polytheism) that expels one from Islam and necessitates eternal damnation in Hell. Allah Almighty says: "So do not invoke with Allah another deity, lest you be among those punished." Ibn Taymiyyah said: "Whoever makes angels and prophets intermediaries, supplicating them, relying on them, and asking them to bring benefits or repel harms, is a disbeliever by the consensus of Muslims." However, a specific individual is not declared guilty of major shirk until proof has been established against them, as they might be ignorant or have a misinterpretation, and the judgment belongs to the Sharia court. As for intercession in the Afterlife, it occurs only after Allah's permission and His satisfaction with the intercessor and the one interceded for. Moreover, both the intercessor and the one interceded for must be among the people of Tawhid (monotheism) and sincerity, for shirk does not benefit its adherents through intercession.