This has been noticed by several commentators:
Dust shall be the serpent's meat. Here we have a new feature, not contained in the earlier description. Serpents shall become harmless,
anal instead of preying upon beasts, or birds, or reptiles, shall be
content with the food assigned them in the primeval decree, "Upon thy
belly shalt thou go, and dust shalt thou eat all the days of thy life"
(Genesis 3:14). Mr. Cheyne appositely notes that "much dust is the
food of the shades in the Assyrio-Babylonian Hades" (see the "Legend
of Ishtar" in the 'Records of the Past,' vol. 1. p. 143, line 8).
And dust shall be the serpent's meat - There is evidently here an allusion to the sentence pronounced on the serpent in Genesis 3:14.
The meaning of the declaration here is, probably, that dust should
continue to be the food of the serpent. The sentence on him should be
perpetual. He should not be injurious to man - either by tempting him
again, or by the venom of his fangs. The state of security would be as
great under the Messiah as if the most deadly and poisonous kinds of
reptiles should become wholly innocuous, and should not attempt to
prey upon people.
Jamieson-Fausset-Brown Bible Commentary
and dust—rather, "but dust," &c. The curse shall remain on the serpent
[Horsley], (Ge 3:14; Mic 7:17). "To lick the dust" is figurative of
the utter and perpetual degradation of Satan and his emissaries (Isa
49:23; Ps 72:9).
Thus, it appears that the perpetual curse of the snake (Gen 3:14) is to be a reminder of the complete triumph of God and Jesus over the great serpent, the Devil. This is presumably similar to the persistent scars of Jesus as displayed in John 20:24-29.
However, some commentators also take Isa 65 & 66 as entirely figurative and thus the perpetual curse on the serpent as figurative - I do not subscribe to this view as it would severely distort the entire narrative of Isa 65 & 66.