It was probably around the early 300's AD/CE, when the Roman Forum began to fall into disuse. Emperor Constantine had relocated the Roman colonial Administrative Centers to Constantinople in the East-(formerly, Byzantium), as well as to the city of Milan in Northern Italy. By sapping the Forum of its once mighty reputation and sophistication, cities, such as Constantinople, as well as Milan greatly benefited from Rome's increasing demise. (It should be noted that Constantine's Rome was primarily focusing on the earliest construction of a Church encircling the tomb of Saint Peter.......what would become Saint Peter's Basilica 1000 plus years later).
During the 400's AD/CE The Roman Forum became increasingly susceptible to Visigoth invasions from the Germanic lands to its North-(i.e. Alaric), accompaniedfollowed by the arrival of the Mongol-Huns from Central Asia.
When the city of Rome and its Forum collapsed in 476 AD/CE, the once prestigious Roman Forum became a remnant of its not-so-distant glorious past.