This is false in reality, and true in Protestantism (because Protestantism is not reality). It is simply falsehood to say the Jewish canon was closed before Christ.
Well the canon has always been closed, since the time of Deuteronomy at least, God forbid adding to scripture. It’s always been closed.
But that didn’t mean God couldn’t add, it’s man who cannot add.
Sure, there were discussions about the canon over the years. But then does a discussion mean the canon is open? If so, I would say this Forum would prove an open canon of scripture today. The canon must have been open during Vatican II, after all they discussed canon, right?
The canon today is the same as the Pharisees used at the time of Jesus, it hasn’t changed. The existence of sects that didn’t agree or that there were discussions doesn’t change that fact.
We don’t have good conclusive evidence for it, and that leads to a lot of speculation which is necessary for any conclusion that would say definitely canon was set by a particular group or at a particular time.
Now for evidence that the canon was set or at least that the canon was pretty well set at the time of Jeremiah, I would refer you to Sirach. The books of Maccabees have evidence that leads some to say that it was they who set a canon.
But I guess to you, Sirach and Maccabees are simply liars. Not evidence to be weighed, but simply to be ignored because you’ve been told to believe otherwise.
I could understand why the imperative in 4 Esdras to stick to the 24 books for most people, would be ignored by you, after all, since it became customary after Trent to drop it completely it’s nothing, not even evidence.
But I guess since I read and study those books, well that just makes me a liar.