if you want real authentic then it really depends of what version you are playing of vanilla, that’s a complex problem; where you slate that. I don’t think auto down rank was a thing tho until 2.o (tbc prepatch).
Previous to TBC you had to manual select your buffs.
Other interesting rules regarding friendly buffs was before TBC prepatch you could buff any friendly NPC who’s not hostile, this function was eliminated in TBC to prevent griefing of enemy players. Again not authentic to Vanilla and should never have been part of Classic Era or Classic 2019.