JFYI, くびす is indeed listed in dictionaries, but it's an obsolete word used in the premodern era.;-) Instead, きびす is used now, basically only in some set phrases such like きびすを返す "to turn on one's heel". Anyway, neither くびす nor きびす is used as the meaning "heel" in daily life. かかと is the one.
Ah, OK, Tori-san... I kinda knew that. The dictionary which I used as source marked that word as "restricted meaning" or something like that... I put it here just because I thought it different and uncommon. Anyway, it's great to have such an enlightenment as you did. Thanks as always. :emoji_thumbsup:
くびす/クース の す