Basically, the KJV "ready" is equivalent to the JPS "condemned." Ready had a different since in the KJV. It means they were in line to be slaughtered, not that the people were ready to die. Apparently, because "ready to be slain" was confusing to the modern reader, NKJV went with "stumbling to the slaughter."
The issue is translating the word וּמָטִ֥ים. This is variation on how to translate it and even an issue about whether the root is מָטֶה or מוֹט. Considering the English of the KJV, it is actually similar to the JPS translation. The poetic language doesn't help.
The senses of the verb from Logos Bible Software.
מוֹט ... vb. totter, shake, slip (usually poet.)...Pt. מָט Pr 25:26; pl. מָטִים Pr 24:11;...
Brown, F., Driver, S. R., & Briggs, C. A. (1977). Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (p. 556). Oxford: Clarendon Press.
If you refrained from rescuing those taken off to death,
Those condemned to slaughter
(Prov. 24:11, JPS Tanakh)
Rescue those who are being taken away to death;
hold back those who are stumbling to the slaughter.
(Prov. 24:11, ESV)
If thou forbear to deliver them that are drawn unto death,
And those that are ready to be slain;
(Prov. 24:11, KJV)
ῥῦσαι ἀγομένους εἰς θάνατον καὶ ἐκπρίου [you are sawed off] κτεινομένους, μὴ φείσῃ, (Prov. 24:11)
erue eos qui ducuntur ad mortem et qui trahuntur [he is dragged/hauled] ad interitum liberare ne cesses (Prov. 24:11, Vulgate)
Deliver those who are drawn toward death,
And hold back those stumbling to the slaughter.
(Prov. 24:11, NKJV)
Rescue those being led away to death;
hold back those staggering toward slaughter.
(Prov. 24:11, NIV84)
Libra a los que son llevados a la muerte;
no dejes de librar a los que van tambaleando [staggering] a la matanza.
(Prov. 24:11, RVA)
Save those who are being led to their death;
rescue those who are about to be killed.
(Prov. 24:11, NCB)