The canon refers to the 66 books we have in the Bible. Not only that but it also says that all matters of faith and doctrine can only be based upon the canon. Biblical canon wiki

Most Christians regard the “canon is closed“, that is, no books can be added, nor taken away from it, neither modifications to existing ones.

Regarding “the canon”, i.e., scripture, I cannot say it is closed or not, but it does appear that the little book that John was told to “eat” in Revelation 10, and then testify to the world about it may indicate a summarized gospel message during the Great Tribulation period. Obviously, there are many interpretations to this section in Revelations, but it illustrates a case where the  “canon” is not necessarily  closed.

Furthermore, it may be a stretch, but consider the amazing spiritual inspiration we get from the Christian hymns developed during the Church Age. See how that doxology has helped our understanding of theology that we get from the canon. In this context, the canon could be said to have never closed. Consider these biblical spiritual words of inspiration in “How Firm a Foundation”:

How firm a foundation, ye Saints of the Lord, Is laid for your faith in his excellent word!

What more can he say than to you he hath said, Who unto the Savior, who unto the Savior, Who unto the Savior for refuge have fled?

In ev’ry condition—in sickness, in health, In poverty’s vale or abounding in wealth, At home or abroad, on the land or the sea—As thy days may demand, as thy days may demand, As thy days may demand, so thy succor shall be.

Fear not, I am with thee; oh, be not dismayed, For I am thy God and will still give thee aid. I’ll strengthen thee, help thee, and cause thee to stand, Upheld by my righteous, upheld by my righteous, Upheld by my righteous, omnipotent hand.

When through the deep waters I call thee to go, The rivers of sorrow shall not thee o’erflow, For I will be with thee, thy troubles to bless, And sanctify to thee, and sanctify to thee, And sanctify to thee thy deepest distress.