The book I normally trust regarding these things does claim thatHmm, Compendium says BIOS/XBIOS/GEMDOS "may utilize" d0-d2/a0-a2. So maybe it isn't always needed but maybe on some obscure version of TOS it is.
d0-d2/a0-a2 are indeed clobbered for BIOS and XBIOS, but that only
d0-d1/a0-a1 are for GEMDOS (and actually only a0 for everything except
SUPER).