The KJV says "him" in the margin. Clearly the KJV believes the promise of preservation is to a person/people, not the Word in v. 6
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Umm, you are drawing your attention, not on the text which is in the final analysis that has been rejected by translators of the KJB and still can be contested the word "him" is "it". It would be the literal way of saying him (singular) can be regarded as the neutral "it" about the "words of the LORD" by its immediate context being plural. iOW, "him" was considered and has been discussed or debated by the KJB translators but they have not put it in the text as their final analysis demands so.
Contextually wise, v2, was about “words” such as “speak in vanities” “flattering lips” “speak”
v.3 again we have “flattering lips”, “tongue speaketh”
v.4 same as verses 2-3
Now, in v6, the writer is contrasting the words of the wicked and vile men from the words of the LORD as he promised to keep his words to those poor or the needy.
For the neutral word "it", my companion "The Shorter Oxford Dictionary" traces the word from "hit" which has nominative masculine He or male sex. During the Middle English
hit lost its initial
h. .
He or “him” is identical to
it (pp.965, 1,120)