[kens]

"Astral plane" predates Dungeons and Dragons by centuries. Looking at old discussions, I couldn't find any evidence that Unicode's usage is connected with D&D.

Early discussion of "astral character" or "astral plane" for the Unicode supplementary planes at: https://unicode.org/mail-arch/unicode-ml/Archives-Old/UML024... Even earlier 1998 use: https://www.unicode.org/L2/L1998/98354.pdf