Actually, it was not the church in Jerusalem which fell in AD 70. It was the Jewish community and worship which fell, especially with the destruction of the temple, which was burned and leveled when Roman General Titus and his troops overthrew Jerusalem.
Yes, this was a prophecy by Jesus of the overthrow of Jerusalem by Rome in AD 70. Specifically, Matthew 24:3-28, as well as Luke 21:5-24 are prophecies by Jesus of the destruction of Jerusalem which happened almost exactly forty years after the prophecy. Also, in Matthew, Jesus tells us that what he is prophesying was also prophesied by Daniel in Daniel 9:26-27, describing the “Abomination of Desolation.’ This is probably a reference to the pagan sacrifice that Titus offered on the site of the destroyed temple. Josephus, in his Jewish Wars, tells us about this abomination of Titus.
About the Jerusalem church taking Jesus’ advice and leaving, we do not have contemporary accounts of this as far as I know. Josephus does not specifically mention the church fleeing to the mountains. However, the church historian Eusebius does mention the flight of the church in AD 69, when the siege began. He is quite specific, telling us that the disciples fled to the city of Pella, where a mainly-Jewish Christian church was established at that time. Eusebius wrote nearly three hundred years later, so we may want to be a bit cautious about using this as proof, but he does quote earlier sources and he is generally (but not 100%) reliable, so it is very likely that this is what actually happened. It is encouraging to learn that the church was saved from the destruction which God brought on Jerusalem because they listened to the prophecy of Jesus.