^16:7 And I heard the altar respond:

“Yes, Lord God Almighty,
true and just are your judgments.”

And I heard the altar respond:

John indicates the altar is speaking, but other translations indicate it is another angel or the collective voices of the multitudes underneath the ALTAR (Revelation 6:9-11).

This is the second "witness" to the true and righteous judgments of the ALMIGHTY. The first witness was the "angel of the waters" (verse 5). Thus the Matthew 18:16 requirement to establish the truth is fulfilled.

“Yes, Lord God Almighty,
true and just are your judgments.”

The phrase "THE ALMIGHTY" is used often in the Revelation. See the list in Revelation 1:8.

Later, in Revelation 19:2, the GREAT MULTITUDE of heaven confirm once more that GOD'S JUDGMENTS are true and righteous. This principle is recognized in the Old Testament in Psalm 19:7-9-14 and Psalm 145:17-21.

Do not think these judgments end when the fourth bowl is poured out. This judgment goes on forever just as the fire on the altar was commanded to burn continually (Leviticus 6:8-13).