An em dash is used to indicate (among other things) an abrupt change of direction or an interruption within a sentence. Neither is present in the first sentence of the example, so the dashes should be replaced with commas.
The phrase sixteen years ago is nonrestrictive (you could get rid of it without losing the basic meaning of the sentence), so the rest of the sentence must make sense grammatically even without it. You wouldn’t write You are one of many prisoners who was sent…, would you? Change was to were.
I know what the writer was trying to say with interstellar planet, but that’s not the right term. Change interstellar planet to exoplanet.
You are one of many prisoners who, sixteen years ago, were sent from Earth on a generation ship to colonize an exoplanet. After years of living in squalor, you discover the generation ship was just a reality TV set and that you have been duped all these years.
(I think I tried to watch this television series once…)