On the reverse side of this question, that is, proofs that the "great crowd" is in the New Covenant:
'They wash their robes and make them white in the blood of the Lamb.' (Rev 7:14)
Compared with:
Also, he took a cup and, having given thanks, he gave it to them, saying: "Drink out of it, all of YOU; for this means my ‘blood of the covenant,' which is to be poured out in behalf of many for forgiveness of sins. (Mt 26:27, 28)
They take advantage of the blood of the Lamb in the very way that Jesus said the "blood of the covenant" does. In this point in the coversation at the last supper, Luke 22:20 uses the phrase "new covenant."
And then there is:
They will hunger no more nor thirst anymore, neither will the sun beat down upon them nor any scorching heat, 17 because the Lamb, who is in the midst of the throne, will shepherd them, and will guide them to fountains of waters of life. And God will wipe out every tear from their eyes. . . (Rev 7:16, 17)
Compared with:
This is what Jehovah has said: "In a time of goodwill I have answered you, and in a day of salvation I have helped you; and I kept safeguarding you that I might give you as a covenant for the people, to rehabilitate the land, to bring about the repossessing of the desolated hereditary possessions, 9 to say to the prisoners, ‘Come out!' to those who are in the darkness, ‘Reveal yourselves!' By the ways they will pasture, and on all beaten paths their pasturing will be. 10 They will not go hungry, neither will they go thirsty, nor will parching heat or sun strike them. For the One who is having pity upon them will lead them, and by the springs of water he will conduct them. (Isa 49:8 - 10)
Rev 7:16, 17 is an allusion to Isa 49:9, 10. Isa 49:8 refers to Jesus as being made "a covenant for the people." The "people" under that "covenant" are then described in verses 9 and 10.
Isa 49:8 is partially quoted by Paul in 2Cor 6:2 and given as a call to the Corinthians to respond. The "prisoners" (Isa 49:9) that respond 'repossess the ... hereditary possessions.' They are obviously being referred to as a people belonging to the covenant that granted them "hereditary possessions" in the first place. Since they have "hereditary possessions", they are "sons." (Compare Gal 4:7)
It's interesting how the Society's Isaiah commentary deals with these verses in Isaiah 49. They take Jesus as being referred to in verse 8, but they substitute "pledge" for "covenant." But never explain what they mean by "pledge." They are trying to remove the idea of a "covenant" with regard to the great crowd.
I hope this adds some to the discussion
Take care