A viewer continues: “Is the thought plane part of the astral plane?”

This is not so, strictly speaking, it is a repository of consciousness that is local to the person. It is in a higher vibrational state than the physical being and, in a sense, is contiguous with the lower astral plane but is a separate compartment of its own without a true blending. So it is not really accurate to consider it as residing within the lower astral plane because that is defined as an energetic level harboring discarnate entities and the fallen ones, the dark spirit meddlers, so an aspect of consciousness that is a part of normal functioning does not reside in a dysfunctional zone of disarray, incompletion, and corruption.