It is highly questionable whether there was actually a council at Jamnia. Ever.
And to call it a “council” suggests some sort of correlation with a Catholic Ecumenical Council. The Jewish faith has never had (and still does not have) any concept of earthly authority comparable to the Catholic Church. There’s no such thing as a Jewish ecumenical council. If this “council” was held, it was merely a meeting of local rabbis who lacked any sort of authority to define anything (including the Jewish canon).
The Christian Scriptures were all written within a short time frame (within the Apostolic Age), but the Jewish canon was written over several centuries, and writings were included (or not) at the discretion of local rabbis. Over time (centuries) a consensus developed, and certain writings were recognized as “canonical” by a consensus of rabbis.
Following the Babylonian Captivity (and including the Apostolic Era) the large majority (perhaps 90%) of Jews spoke Greek as their first language (these were the “Hellenistic Jews”). Hebrew (Aramaic, etc) was uncommon beyond the Holy Land.
From the Fourth Century BC, additional writings were gradually incorporated into the ever-evolving Jewish “canon.” Since most Jews spoke Greek, these writings were originally written in Greek. Since there was no actual Jewish authority to define the “canon,” there were regional variations. This is why there are variations between the OT in the Western and several Eastern Catholic Churches. Early Christian Churches simply adopted whatever OT was in use by the Synagogue down the street, but it was possible that no two Synagogues had the same idea of “scripture.” Thus, Christian Catholic Churches have some disparity regarding the OT. But this has NEVER been a source of contention.
Jesus came and went. Most Jews hardly noticed.
But, in 70 AD, the Second Temple was razed by the Romans. Jews noticed that.
Here’s where the “council of Jamnia” comes in. According to the myth, the Jews were shocked by the destruction of the Temple, and met together in AD 90 to figure out what they had done wrong to deserve such wrath from God. According to the myth, the “council” decided that they had become too pagan, especially by incorporating scriptures written in a pagan language (Greek). According to the myth, they threw out all of the Greek stuff and closed the canon to include only the Hebrew writings composed before the Babylonian Captivity.
There is SOME truth here, but the idea that the Jews convened any sort of ecumenical council has no historical basis whatsoever. The Jewish “canon” continued to evolve well past 90 AD, but there’s no doubt that the Greek writings were falling into disfavor and were gradually (over several centuries) excluded.
Around the Seventh Century (and continuing for another three centuries) an influential group of Jewish rabbis known as the Masoretes compiled their version of a Jewish “canon.” This became known as the Masoretic Text (commonly cited as MT). This “canon” excluded all Greek OT writings.
Over the next few centuries, the MT was gradually adopted by most Jewish communities. By the Thirteenth Century it would have been difficult to find a Jewish community that did not regard the MT as somehow “authoritative.” This version of the Jewish “canon” is still regarded as authoritative" in our time.
Even IF the Jamnia myth was true, it would have no bearing on any Christian Catholic Church (East or West). Jesus endowed the Catholic Church with actual authority (something that God never did for the Jewish faith). What Jews did (or did not do) in 90 AD would have been irrelevant to the Catholic Church. What Jews actually did in the Seventh Century is equally irrelevant. Catholics have no problem with Scripture that was originally written in Greek, since ALL of the New Testament was originally written in Greek (except, perhaps, the Gospel of Matthew).