otherwise, slavery was common in human history.
i say that as the daughter of indentured irishmen.

Nor even the time period. Enslavement of the "Other" and the more readily exploited continues to this day--including in this country. Anti-immigrant bigotry and policies have enabled it--especially in the sex trade. Those who whitewash our past cannot bury what is ongoing.

It is not a recent behavior.

While I certainly believe that the Romans used slave labor in their works in G.B., why assume that this individual was a slave?

Perhaps the young man was a mass murderer, who is to say that he was not an insolent soldier? A slave, cobbled up like that, would have been useless for work. which is generally the purpose of slavery. I would also wonder why the expensive shackles would not have been retrieved.

I've only heard about Roman slaves being buried with other enslaved people, and buried with items for the afterlife, like pottery and food. Meanwhile, I've heard that certain criminals were just thrown into pits. This person was shackled and in a pit. I'm guessing they were a criminal.

not shackles on their legs. Likely this guy was a criminal, a war captive, or some other miscreant about to be sold into slavery in a truly rotten job like Roman mining, where life span was often measured in days. Unless there's an obvious COD, it's likely he threw himself into that ditch, drowning being preferable to anything else he'd be likely to face.

metal was thrown away.

One of my most terrifying moments was seeing real slave chains at an Indiana flea market. I wish I had bought them to show others. No one can comprehend the vicious deep cruelty without seeing the real thing.