Since when did the Bible become sealed?

I was just reading the Psalms, and it occurred to me that this book was written by somebody at one time after Jesus’s death. Why was the Bible able to be edited and added to back then, but now it is completely sealed and unable to be augmented and/or modified?

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Recommended Reading:

[LIST]
]The Old Testament Canon
]How to prove deuterocanonical books are inspired?
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Did Catholics add books to the Bible?
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]* Original books of Bible**
*]Could a book be added to the Bible?
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