The Malbim sees the covenant as one with the kingdom of Assyria. Other commentators see it as a metaphorical covenant with death to protect the people. While the Metzudat David says the scoffers are ones who mock the prophet and his message from God, the Radak points to the leaders of the cities of Judah, specifically:
אמר כנגד שרי יהודה שהם כמושלים על העם הזה כי אחז המלך לא היה עושה התועבות ההם אם השרים היו מוחים בידו אבל כשהנביא היה מוכיחם ומודיעם הפורענות העתידה לבוא עליהם היו מתלוצצים ואומרים כרתנו ברית את מות
The
"leaders of the cities of Judah", the allegorists of this people (moshlei), the scoffers (Isaiah 28:14), would be the scribes and the Pharisees. And which "prophet, who purposes to "teach knowledge" (Isaiah8:9) is
mocked"? Is it the prophet promised by Moses, which Israel was supposed to listen too? As for Assyria, they were a problem for the house of Israel, not the house of Judah and their home of Judea, at least until Zechariah 14:1-3 & Isaiah 7:17-20. The "covenant with death", was a covenant made with "Sheol"/grave, not Assyria.(Isaiah 28:18). That new "covenant with death", is called the "gospel of Grace", proffered by the false prophet, and Pharisee Paul, and entails an escape from death, whereas his followers would not taste of death. It now has about 2 billion followers, a good portion who have already died.
Isaiah 7:17-20
"Now therefore, behold, the Lord is about to bring on them the strong and abundant waters of the Euphrates, Even the king of Assyria and all his glory; And it will rise up over all its channels and go over all its banks. "Then it will sweep on into Judah, it will overflow and pass through, It will reach even to the neck; And the spread of its wings will fill the breadth of your land, O Immanuel.
Isaiah 10:5
Woe to Assyria, the rod of My anger And the staff in whose hands is My indignation,
Isaiah 10:12
So it will be that when the Lord has completed all His work on Mount Zion and on Jerusalem, He will say, "I will punish the fruit of the arrogant heart of the king of Assyria and the pomp of his haughtiness."
Isaiah 11:11
Then it will happen on that day that the Lord Will again recover the second time with His hand The remnant of His people, who will remain, From Assyria, Egypt, Pathros, Cush, Elam, Shinar, Hamath, And from the islands of the sea.
Isaiah 11:16
And there will be a highway from Assyria For the remnant of His people who will be left, Just as there was for Israel In the day that they came up out of the land of Egypt.