Eh, that being said, the translation isn’t strictly necessary, even in that Septuagint translation. It isn’t only limbs being buried in Gehenna in the context of Isaiah 66, but whole bodies – though whole dismembered bodies – of the pagan armies from whom YHWH rescues impenitent Israel (this being what finally leads the Judean Jews still rebelling against God to swing around back into being loyal to Him.)
Still, pieces are buried there, too. In a parallel scene from Ezekiel, one of the penitent duties of newly reformed Israel is to send out squads to find and bury the bodies of their enemies; some of those squads just go out with flags along the road and into the countryside to mark discovered remains, so that another squad can come back and properly inter their bones in Gehenna’s mass grave, even if it’s only a part.
But the usual interpretation in GosMark and GosLuke works with the terminology: better to cast away body parts now (where to is irrelevant) than to be cast with your whole body into Gehenna. The terms and grammar could mean casting those parts into Gehenna rather than one’s whole body, but it doesn’t have to mean that.
Either way, it’s meant to be a sarcastic rebuttal to people who refuse to take personal responsibility for their sins but blame it on a misbehaving body. And in Mark 9 with Matt’s parallel, it’s meant to be launched at the apostles themselves as a warning that they shouldn’t be pridefully thinking of themselves as being better than other people (the original version having been said to crowds more generally during the Sermon on the Mount/Lakeshore.)